I have done some (inexact) experiments with the values Firaxis exposed in the BarbarianClansMode_GameplayData.xml file, so this thread doesn't have to stumble around in a dataless darkness. Here goes (caveat: this might possibly be more complicated):
BARBARIAN_CLANS_CIV_CONVERSION_POINTS_STANDARD (default 120) pretty much says what it does, it takes 120 points for a camp to convert . (This should - but might not - scale with game pace)
BARBARIAN_CLANS_CIV_CONVERSION_INCREMENT_CHANCE (default 50) seems to describe the chance that the counter for a camp goes up on given turn, so there's some (low) randomness thrown in.
If this event fires (about every 2nd turn, obviously), the counter goes up several points (possibly depending on the amount of food around the camp, I don't have the stamina to figure this out now). The fairly normal camp I experimented on seemed to go up three points, so an early camp would convert around turn 80 without interference, give or take some randomness. Should be less in the tundra, but those seem to convert pretty soon, as well.
Now, to keep modders on their toes, the increment chance seems to be inverted, so camps develop faster with lower values - with 0 they seem to count up every turn, while a value of 100 means that the camps don't develop at all without help.
I have a sneaking suspicion that the five points a camp gets when you buy a unit are counted more than once, but I might be imagining things, as the bar is hard to read. They might also get multiplied with the food value or something else.
Conclusion: the default values mean that camps disappear pretty quickly. So, we could adjust several things here: By lowerings the points total and lowering the chance to advance per turn (though again, this value is inverted!), the randomness of conversions would be higher and the interactions with the clans more meaningful. It might also mean that snow barbarians and other isolated camps convert later (would have to be tested)
We could also simply put the conversion threshold higher, late spawning camps (yes, they exist) and really tough survivor barbs do build some entertaining units later. Obviously the likelyhood that some camps survive to form a city state sinks exponentially the higher the threshold goes.