My DD just turned 4, and she still needs me to lie down with her to fall asleep--she tells me she's afraid to fall asleep alone, and I can understand that (I feel like she's still young).
It only takes her about 5-10 minutes to fall asleep though, so I don't really mind. When it used to take longer than that, I let her know that she could play, talk, roll around, etc. as long as she wanted to in the bed, and that I would be right outside the door and would lie down with her when she was ready to sleep. Basically, I refused to lie with her until she was ready to actually sleep--I never threatened to leave or abandon her, but rather I made it sound really matter of fact ("hey, no problem at all that you want to play--I'll be right outside until you're ready to sleep") And then I'd sit outside the door and read my book until she told me she was ready to sleep. Sometimes it took 3 or 4 tries each night, but eventually she got the idea that when we lie down together, we go to sleep. And now it usually takes 5 minutes.
With my DH, he ends up lying with her for 45 minutes to an hour--he doesn't put any limits, and so it ends up taking forever.
Anyway, that's how I do it with my 4 year old.