Just from this event point of view, you need to get a total of 1,260 fertilizer to max out the tree. I haven't tried yet, but I am assuming that this is like the valentines mini event where if you go over the required amount of fertilizer for that level it doesn't carry over. If that is the case, the total amount of hours it will take to get to level 10 will be:
With all premium characters and Jesse: 144 hours
With all premium characters without Jesse: 231 hours
With no premium characters: 273 hours
If that carry over assumption isn't correct, and you can carry over excess fertilizer, then with Jesse, this will drop to 126 hours. Theoretically, you could get around this carry over issue by staggering by a few minutes and then being really exact about signing back in to get that fertilizer on the next level....though I don't have Jesse, so I probably won't find out...