How long can I expect my 2yr old lab to "hold it". I let her out in the yard (her usual spot) about 9-11pm and assume she can make it to at least 6am. When she comes to get me about 4am, I want to be sure it's okay to let her hang on for a couple of hours. It's hard to tell if she's getting me up for 'gotta pee' or 'hey are you awake yet?' (Every day is a party to her)

She's in excellent health. I haven't checked with the vet yet to see if there's a problem but she's always done this now and then.
My other dog didn't pee for 18hrs once. We had a big storm and it was raining so much she didn't want to go outside.

We kept trying to take her out but she dug in her heels and just wanted to stay warm and dry inside.
Basically, generally speaking, how long can a normal, healthy dog 'hold it'?