It takes a long time, mine became lv 8 after about 3 month of game play and I almost never missed any watering(note u gain exp on it when YOU water THEIR tree and your own tree).
I'd say it takes at least 2 month and a half even if you never miss any.
lv 8 will take 1k exp, which is another 3 month minimal to be lv 9, much fun...