FYI, 7 to 8 is 500 exp (like Niji said, this exp is only shown when you hover your mouse over your tree level, it's NOT the top exp)
This exp goes up by 1 every time YOU water something, you can do 1 self and I believe 10 friend water a day
Thus, minimal is 46 days.
8 to 9 is 1k.... yeah... joy....