1 billion requests is 1 PB / day. There won’t be that many requests (unless the app guys can earn by stuffing their GET requests).
My (guess)estimate is 1% of the capacity/day (or, if there’s 20 PB in raw capacity out there, that’d be 200 TB.)
Routing rewards would reward farmers to build nodes with suitable for caching, which would reduce minimum bandwidth requirements from the rest of the network. But it’d have to be rewarded. Imagine getting a 10% cut for “saved” GET. Maybe you’d buy one of a cheap SSD drive and keep SAFE cache on it.