Transiting Neptune conjunct your Jupiter can only happen once in a lifetime, if at all. Neptune takes about fourteen years to travel through each of the zodiac signs. Currently it is in the middle of Aquarius.
Both Jupiter and Neptune have a lot to do with spiritual interests and the expansion of consciousness. When Neptune goes over your Jupiter, be prepared for some kind of expansion of consciousness. Perhaps you will become interested in religion for the first time, or you all of a sudden be running into all sorts of mystical types of people. Or spiritual people. Maybe you will begin or intensify a study of eastern philosophies or metaphysical subjects.
Several astrology authors note about this transit that it can bring great material as well as spiritual rewards. But the material rewards, if they were to come, would come without effort during this particular transit. The real opportunity here is to focus on the transpersonal and the higher spiritual realms.
If you are an artist by temperament or profession this could be a tremendously inspiring time for you.
The most uplifting description I've seen of this transit is by Grant Lewi in Astrology for the Millions. He writes:
"A period of unparalleled opportunity, when intangibles work on your side to bring new chances for material and spiritual expansion. Do not permit your life or your viewpoint to get into a rut at this time. Your chances are best in proportion to the height of your aims and the idealism of your aspirations. Release yourself from materialistic thoughts: practical rewards will be "added unto you" as you devote yourself wholesouledly to broad social, artistic or humanitarian aims. Be ready for unusual and spectacular developments; widen your horizon to take in everything possible. In its worst manifestation, leads to physical excess, extravagance and self-indulgence of a particularly heady and disagreeable sort. But this will come to only the material and sensational-minded. To the average run of humanity, halfway between heaven and earth in their outlook, this is a never-to-be-repeated chance to interpret the highest of idealism in the practical realm of everyday life, so as to achieve permanence of both philosophy and security."
The above fine art print is "Buddha" by Author Unknown