Part 1 and 2 are part of the same season, there was no break in between. Between 2 and 3, there was a one year break. Between 3 and 4, 9 months. Between 4 and 5, a Year and 9 month.
But the thing is, we can't know. There is even the possibility that the rest won't be adapted (because Part 6 will have a copyright issue and Part 7 won't be able to be animated at the same rate as the other parts, Part 8 has no problems though)
Realistically, I think we could expect Part 6 for Fall 2020, that would be a year and 3 month of break. Part 7 will take way longer.
It's the longest part, and by far the hardest to turn into an anime. Assuming Part 6 ends in Spring 2021, I don't see 7 before 2023, and it might run for 48 episodes. And so the best solution would be to take a break in the middle of the part, which would push part 7's end to 2025. Then, Part 8 in 2026-2027. By then, Part 9 will probably have started and ended. And if it is indeed the final jojo part, then the jojo anime could end forever in 2029.
TL;DR Around 10 years of Jojo anime to go !
