Would 65C be too high?
Have you actually checked that it gets anywhere near this temperature? I have not maxed one out all night, but I occasionally do it for 30+ minutes while observing the core temperature, and even after an hour it barely rises beyond 50C. In fact, it seems to me that it stabilizes at that point.
This is in a 20-25C room, no overclocking, no heatsinks fans, etc. I may have done it with a B overclocked to 800 or 900 Mhz as well.
You should explore this before you assume that running at 100% will cause a steady linear rise in core temp. I do not think this is the case; it is more likely to be roughly logarithmic and plateau.
Also, damage due to temperature in electronics is primarily caused by either:
Exceeding a certain maximum, at which point internal components fuse. For the pi the OEM states this as 85C.
Oscillating expansion and contraction. In other words, if you can get it up to 65C (which probably requires overclocking and/or a high ambient temperature) by maxing the processor out for 3 minutes, and you do this, then allow it to cool back down, then heat it up again and so on, you are producing the greatest possible wear on the component. However, if the temperature stabilizes after 3 minutes and you leave it there for 4 hours, then this is not substantially worse than leaving it plugged in doing nothing for 4 hours. It is not expanding and contracting constantly.
There's a bit of idealization in the last point -- a pi left doing nothing will probably last longer than one constantly maxed out -- but the general principal holds.
What temp would you feel comfortable running your pi at all night?
Since pis are inexpensive and I buy them to use them, if that use included a desire to do intensive calculations all night I'd say whatever temperature it plateaus at. There's a related question about overclocking, since that will increase the temperature, and the question becomes largely subjective.
Personally, if it plateaued over 70C I would reduce the overclocking. This is mostly because laptops I've owned that ended up with overheating problems (because I let too much dust pack in) would get that hot repeatedly without damage, and my current laptop core temp seems to have the same idle temperature as a pi (45C). I think people who do a lot of desktop gaming will run systems with a lower idle temp (30-35C) at 70-80C for hours at a time. This is something you might want to explore by searching overclocker and gamer forums.