From personal experience with tab counts into the hundreds, I'd guess about 800+ MB. When you load the browser up that badly, windows start disappearing. Then they'll suddenly appear when you close one, or cycle through the ones you can still see..Wow. How much RAM is that using?
I don't perceive hundreds of tabs open as being that unusual. Shopping in a half dozen different vectors, for a dozen different items with comparisons. Multiple forum tabs, in multiple forums, left unattended, or waiting for replies. A couple of Email accounts, and last, but not least, a couple hundred examples of art and erotic photography.
Of course my normal internet machine only has 1.5GB of memory, so it's fairly easy to "hit the wall", so to speak.
If it wasn't DDR, and I didn't have to buy 2 sticks of at least 500GB to get to the deuce, in a 7+ year old computer, I might do something about it.