At the rate it is going, I do not think the show will be pulled off the air.. At least not for a LONG time.
I hope it sticks around for a few more years. I feel kind of ashamed that I've only gotten into it once Season 2 was done. I'm going to be moving out on my own at some point and I'll either be going military or post secondary or both, and I'm going to need something to pull through those years, and I'm hoping FiM will be there to satisfy my heart.
Meh, whenever it is pulled off the air its popularity will drop really fast. But question is when will they take it off? I imagine that the targeted audience (5 year old girls) has been roughly the same for the past 20 years, so it may depend on how many bronies watch it.
I don't think it'll drop "fast", but all good things must come to an end. I think even once it's finished, the fandom can keep continuing for a few years without the show.