I do not know the answer to your question, but I do know this: I noticed a sharp increase in my libido since I started eating paleo. From an evolutionary perspective, it makes sense that healthier people have a stronger sex drive.

Given that the women gave birth about every 3-4 years and were lactating in between, and considering what lactation and anovulation tends to do to female libido.... well. However, this report suggests otherwise. So non-pregnant non-lactating fertile women probably were at it pretty frequently til they got knocked up.

However - I strongly believe that paleo couples didn't form strictly monogamous, long-term pair bonds, like people today are expected to do. Why do women so often lose interest in their partner, sexually, after a few years? If I'm a paleo woman, it makes NO sense at all to keep having my kids by the same guy - if a disease goes through the tribe and my baby-daddy and therefore offspring aren't immune, all my descendants are dead and my good-monogamous-faithful-wife genes aren't going anywhere fast. Also, with relatively many women dying in childbirth and men dying from injuries, more pair-bonds would be broken up by partner death.

Naaah, I think that paleo humans often had short-term (2-4 years) monogamish love pairs that often split at some point after the kid was born, eg. around age 3-4 when the kid could be left a short time, was moving onto solid foods and therefore the father wasn't 100% vital. If relationships developed into long-term pairings, they were likely semi-open. Alpha males may have had a primary parntner and girls on the side (remember women in relationships with less-masculine guys are more attracted to masculine, alpha, types during their most fertile time. Beta males get the girl by being caring and dependable, alphas by being sexy...)

I also have a sneaking suspicion that we became a little more monogamous after the agricultural revolution - because it was then we got richer and poorer, and material possessions to pass down, and childrearing would have shifted from communal to couple - in which case monogamy would benefit the man (who could pass down possessions with confidence) and the woman (who might well need a 2nd parent when childrearing isn't done communally by the tribe) Our metabolism might not have changed much since the paleolithic but that isn't necessarily the case for sexual strategy, as these can change fast. It's just a theory, though.

In all I'd be willing to hazard a guess that quite a few paleo guys/gals got pretty frequent sex due to a) less libido-killing long term relationships and b)more open relationships/casual sex (so when primary partner is knocked up and not into it, the guy might go elsewhere). But the less attractive guys could have got very little sex (worse than now, because it's entirely possible that a paleo '3/10' lady would rather have a casual shag with a '9/10' guy than a serious monogamous relationship with a '3/10' guy, whereas monogamy/marriage results in more of the opposite. That's changing now, though, now that it's possible for women to be single parents an have their kids survive)

Duration? Don't really see the need for marathon sessions - particularly if you're hooking up with a partner who's not your 'primary'...