How long?

How long do you leave a toddler in their crib before deciding that they are not going to take a nap? Today Evelyn was in her crib for an hour and a half before we gave up. Mind you she was not crying she was alternating between talking and being quiet. Usually she would be quiet for a while then something would happen (mommy going outside, phone ringing ect) then she would talk again for a while. Before putting her down I asked if she were ready for her nap and she said yes and went over to the crib, I guess she just wanted to get in. She did sleep in today until 8 which never happens and on a weekday we would not let happen (and If I had woken up first it wouldn't happen either but since I keep waking up in the middle of the night I'm surprised I even woke up at 8 since she was chattering and playing but not crying. So it looks like today is a day without a nap and I need to get out that book on sleeping again to look into the toddler years although I know my first mistake was letting her sleep in.