Well, in theory, vaginal lubrication lasts as long as you (a woman) are aroused.
When you're turned on, and like really turned on, your body creates more lubrication to prepare for more penetration. Your labia also gets wet because blood is rushing to the genitals and now your vulva is starting to make extra moisture.
So cam girls who start the show wet as hell, probably have some prep time before the show starts.
Other camgirls just get wet by flirting with all these guys, and by activating the vibrator.
And you might notice they sometimes take breaks. Either by stopping the toy, or by going off-camera, or just chatting instead of touching herself.
And believe it or not, some camgirls may even get dry after a while - which is why they use lots of lube either on camera or off-camera.
One last thing: Everyone needs to use MORE lube even if sex just lasts a few minutes. Going "natural" for minutes or hours is going to be taxing.
So definitely add lube when in doubt, as I'm sure most camgirls do. (I used to do camwork and yes I always used lube)