I think that I would be comfortable waiting a pretty long time as long as the baby seemed like it was doing well -- with reactive, normal heart tones.
One of the hospital-based midwifery practices I work with as a doula frequently has issues with interventions that cause long, drawn-out labors (like encouraging moms to come into the hospital way, way too early and encouraging pushing when moms are at full dilation instead of waiting for an urge to push). I have been at a few births with them where the moms pushed for three hours or so and then got the baby to about +1/+2 or so. At that point, the midwives declared the moms too tired to finish pushing, ordered an epidural, had the mom take a 4-8 hour nap, and then start pushing again with the mom well-rested and the baby at about +3. I haven't seen a baby have trouble with that plan yet. Even though I hate the habits that get women into that situation in the first place, I think this strategy has kept them from a few c-sections that other practices might have ordered in the same situation.
As a homebirth midwife, moms in my practice push when they are ready and if they only want to push every few contractions, that's ok, too. Most of the time women push actively by the time the baby is crowning, but I have had women fight strongly not to push and their babies were born anyway, too.
) as we very rarely do any vaginal exams. We just know that as long as mom and baby are doing good, things will happen on their own time. Our bodies and our babies are so very wise. 
