Depends on the pack. There are some packs I have tried that weren't comfortable carrying even 10lbs. Assuming an adequate suspension system I have two set points:

Comfort = Body doesn't hurt: 30-35lbs

If I keep my days to under 20miles, say 6k ft elevation change, I can make it through the day without an lasting pain and repeat days on end. If I try to push for longer days or carry more weight I end up feeling fatigued and the next day I will often hurt.

Comfort = No impact to my perfomance: 18lbs

I found that if I keep my pack weight below around 10% my lean body weight that carrying the pack doesn't seem to impact how long/far I go. On long summer days this means I can crank out 30-40 mile days with 6k ft elevation changes (or shorter distance with more elevation changes) and have energy at the end of the day, or go 20 miles and be able to spend the late afternoon and evening playing hard. This is basically the same as what I could do if I didn't have to carry a pack.

Years ago I read a study funded by the military looking at the impact of pack weight to soldier performance. I can't find the paper anymore, but my memory is that they found that 10-12% lean body weight was where the pack had a measurable impact on how quickly people got fatigued. Steve Sergeant sent a note to the backpackinglight mailing lists which is very similar to a study I remember seeing. His summary was:

A Swiss military report suggests that everyone has a backpack weight threshold at which they become significantly more encumbered. They determined this weight by measuring how much it takes for a person's balance-time to degrade by 20%. You can determine your balance-time degradation by measuring the time that you can stand on one foot without your pack, and then compare that to the time you can do so with your pack on. Apparently the Swiss military sought to optimize the performance of 'light fast' special-forces types. They found that for their typical soldier, balance degraded by 20% when wearing a pack weight between 8% and 10% of their lean body weight. The degree to which the pack carrier's balance degrades directly relates to the rate at which they'll become fatigued. This study suggests ways to improve your backpacking experience. The traditional guideline of 25% to 40% given by some how-to books on backpacking would seem quite high by these standards, so you should try to go lighter. Experiment with loading your pack to minimize the degradation of your balance time.

--Mark