Jupiter mahadasha lasts for 16 years. This is supposed to be one of the most auspicious time in the native's life where marriage, children, education, wealth and gains come into play. This is because Jupiter represents wealth, children, luck, gains, education, learning, teachers, philosophy, religion, long distance travel and our belief system. When one goes through the mahadasha of Jupiter there is a great sense of optimism no matter how hard the road is. Yes, there could be depression, lack of funds if Jupiter is not well placed, education might suffer, but, overall there is always hope. There is always a beckon of light at the end of the tunnel, that's what Jupiter does during it's dasha. It brings hope through the house where Jupiter is situated and the sign he rules like Sagittarius and Pisces. However, is Jupiter's mahadasha good all the time? no, First you have see what houses Jupiter rule, is Jupiter a functional malefic or functional benefic. Then you have to see if Jupiter is placed in kendra or trikona or maraka or even dushtana and who is in cojunction or aspect to Jupiter.
Jupiter can give good result in any house, but you have to see if it's a FM or FB. Like for cancer ascendant Jupiter is a functional malefic, since it rules the 6th house of debt and diseases and 9th house of luck and higher learning. During it's dasha it can give mixed results and it usually does. The native can be in debt, get into accidents, while at the same time progressing in their learning, philosophical views and long distance journeys. If Jupiter is debilitated or sitting in a maraka house then it can provide even less fortune for Cancer ascendant. Another important thing to notice is the nakshatra ruled by Jupiter. Any planet sitting in the nakshatra of Jupiter will also be activated. Nakshatra like Punavasu, Vishaka and Purva Bhadrapada are ruled by Jupiter, so any planet sitting in such constellation will also get activated during the 16 years and provide results according to their placement. This is why mahadasha doesn't just bring only 1 type of event. The native experiences multidimensional problems and good times. Jupiter usually doesn't do well in the 2nd, 7th and 8th house due to these houses being maraka houses as in 'death inflicting' houses, where Jupiter is very uncomfortable and out of his element. Jupiter's conjunction with a malefic or functional malefic planet will also cause Jupiter to lose it's positive effect during the mahadasha.