As far as I can tell you get 1 experience for watering your own tree and 1 experience for watering someone else's tree for a total of 11 xp per day. The lvl 9 tree requires 2000 xp to level so it's going to take 181 days to go from lvl 9 to 10. Level 10 requires 3000 xp to level up, so 273 days.