there is also a timeline issue = the early barbarians won't corss culture boundaries.
The date (more a number of turns, I believe = earlier date at marathon than normal) at which the barbs come for your improvements is level dependent.
I don't know the specifics, but some testing could be done easily with WB.
My understanding is there are several periods of increasing intensity. In order you get:
1. A short period when no barbs are around
2. A period when only animals spawn (animals can't enter cultural borders)
3. A period when barbs spawn but can't enter your borders.
4. The rest of the game, when barbs do everything.
(After some time, say 0AD-ish, I've noticed the spawn rate seems to decrease quite a bit, even if there is still a lot of land that is undeveloped, and so presumably completely in the FOW for all civs. Not sure if there's something in that or if that's just my perception being off).
Obviously, the higher the level you play at, the shorter periods 1,2 and 3 become and the more frequently barbs spawn (or build cities). OTOH at higher levels the AI expands to fill whatever FOW you haven't filled more quickly, so the barb threat tends to disappear far earlier.
I
think (though may be wrong) that setting raging barbs doesn't particularly impact how many barbs spawn, but it does eliminate period 3 (not sure if it elminates 1 too?): As soon as barbs spawn, they can enter your borders. It also means they are more likely to beeline straight to attack your cities, instead of spending time wandering around aimlessly and pillaging (I think in some ways that makes things
easier not harder: I'd rather have a barb sacrificing himself against my CGII archer than pillaging my city into uselessness)