Just pick whenever sounds really good, and isn't more than two seasons. Maybe three if each season is short. If the show turns out to really annoy me, I'll drop it instantly. If it's not grabbing me but seems like it has promise, I'll keep with it until I enjoy it. And if the show reaches a point where it goes downhill, and making myself watch it feels like a chore, I'll dump it, even if I've nearly finished it.