This is another planetary return which can bring substantial changes in life due to the duration of transit and extreme benefic nature of Jupiter. Among all the major planetary returns, we can say that Jupiter Return is going to bring some benefits or positive results in some way. Let's break it in following points -
Planetary Returns - So just for the introduction part, when we are born, every planet is at a particular position. Like, if someone takes birth today then Sun is in Aries, Saturn is in Sagittarius, Moon, Mars & Ketu are in Capricorn, Mercury is in Pisces, Jupiter is in Libra, Venus is in Taurus and Rahu is in Cancer. So, planetary returns are when during transits these planets come into exact same position again in future. We can say that after coming into same position, planet gets energized again for next rotation of the zodiac. Hence, the time period which energizes the planet becomes important. As always, planet with the longer transit duration will have more impact during their returns. So, Saturn Return is most important as it happens after 30 years and lasts for 2.5 years. Rahu/Ketu return or Nodal Return is next which happens after 18 years and lasts for 1.5 years. Jupiter Return happens after 12 years and lasts for 13 months. Mars returns after 2 years and stays in a sign for 45 days unless it is retrograde when it stays for 6 months in same sign. Other planetary returns have lesser importance on their own as they happen every year but they will gain importance if they are happening with major planetary return. Like, if a person is already going through Saturn or Jupiter or Nodal Return then other planets' return during that year gain importance and that year or two becomes significant in person's life.
Jupiter Transits - Jupiter transits in one sign for about 13 months, hence it takes Jupiter about 12 years to come in the same sign where it was at the time of person's birth. So, Jupiter Return takes place after every 12 years and takes about 13 months to give its results. As 13 months is a good long duration, it is immaterial if person is under Jupiter dasha or not. Also, Jupiter matures at 16 years, so anyone over that age can receive benefits of Jupiter.
Jupiter Return - First of all, as Jupiter is naturally benefic planet, it is normal that every transit of Jupiter and Jupiter Return will bring some good results in life, either in personal life or professional life. Overall, there will be something to be thankful and grateful for. Even if Jupiter is debilitated and it brings separation in relationship, after some time person may be thankful for that separation. Also, as Jupiter represents wisdom, during Jupiter Return we get a kind of wisdom about our right path in life. As i said, Jupiter Returns after 12 years of age, so we can go back in memory to remember events which happened at the age of 12 years, 24 years and 36 years and so on. Jupiter also gravitates and expands the results of the house where it is placed. So during Jupiter Return, things related to that house will be expanded and person will feel kind of pull towards that house. Also, person will feel a kind of gratitude once Jupiter Return is over. For girls, Jupiter represents husband, marriage and children too. So, Jupiter Return can bring results related with these areas of life for them. It is also a nice time to educate yourself and learn something new. As Jupiter also represents your Teachers and Guru, Jupiter Return time can bring one such person in life but the genuineness of the Guru will depend on the dignity of Jupiter you may have in your birth chart. So overall, it can be a beneficial transit.
Houses Involved - Again, the most important or impacted area of life will be the house where Jupiter sits/returns in your birth chart. Those things will suddenly become an area of focus. Like, I have Jupiter in 9th house/Cancer and when I was in 24th year of life (2nd Jupiter Return), I started legal practice which is very much a Jupiter & 9th house related things. Also, 9th house is house of higher learning and my interest in all kinds of religious, spiritual, philosophical or literature studies developed at that time only. Then my 3rd Jupiter Return was 3 years back when I took up Astrology Practice as full time profession and many people started considering me as teacher, guide, counselor or even a Guru. It is another indication of someone with Jupiter in 9th house/Cancer. So during both Jupiter Returns, 9th house and things related to 9th house became prominent and I was pulled towards it.
Signs Involved - Then certainly the dignity of Jupiter in birth chart is going to have a say in quality of results one gets during Jupiter Return. So, when Jupiter or Venus are in weak dignity like enemy sign or debilitation, it is not that person will not get any results during their dasha or transits but it is just that person will lose the sense of appreciation of what he receives and remains sulky about what he doesn't have. So, if Jupiter Returns happen in Capricorn or Gemini/Virgo/Taurus/Libra then person will get results related with the house it is placed but not as per expectations. If Jupiter is well placed in own, exalted or friendly sign, then Jupiter Return will bring best of results and person will be able to appreciate it too. So, dignity plays a huge role in the kind of results one gets but again, the results will be based on house where Jupiter Return takes place.
Nakshatras Involved - Then the intensity of results during Jupiter Return transits can be seen based on the nakshatra & degree positions of Jupiter in birth chart. Like, if someone is born with Jupiter in Vishakha Nakshatra then the best results of Jupiter Return in Libra can be seen when transiting Jupiter comes into or near to Vishakha Nakshatra. In this example, person will continue to get results of Jupiter return even when Jupiter goes into Scorpio as Vishakha ends there. All I want to say that best or most intensified results can be seen when Jupiter is in same nakshatra as their birth nakshatra.
Planets Involved - Then of course, if Jupiter Return is taking place over another planet in the sign then that planet results will also be seen as per the nature of Jupiter and the dignity of planets. As Jupiter expands the results, it will expand the results of the planets it is transiting over as per the dignity of those planets.
Conclusion - Overall, we can say that it is a beneficial time period and there is always something to be grateful for during Jupiter Return.