That differs from person to person, and depends on your level of crunch.
Some people start declining after two 50-hour weeks; others can do 60 hours indefinitely without any negative impact on their productivity. Few people get more done in a single hundred-hour week than they do in an 80-hour week, and the vast majority will need multiple weeks of recovery time after those hundred hours to get back to pre-crunch productivity levels.
So I would suggest a few things:
1) Know thyself.
2) Consider whether there's any way you can decrease the crunch by decreasing scope or extending the timeline. Maybe there's a piece of the project that can be done after the deadline instead of before. Maybe there's a component you can simplify. Maybe the deadline is arbitrary and nothing will implode if it actually lingers another two weeks.
3) Is this for your own company or for an employer? Do a gut check, and repeat it every day of your crunch period. Who will really benefit from this project hitting its deadline, and who will really be injured if it doesn't? Is it really worth the risk of long-term damage to your physical and mental health to go all out to complete this project?