In-ground pool with a 1HP pump => probably 40GPM or more on backwash.

5 minute backwash (mfg standard) at 45 GPM = 225 gallons

Watermom, your pool has a smaller pump (1HP AG pumps < 1HP IG pumps) and smaller filter, BUT I'd bet if you look at flow, you STILL are dumping more than 50 gallons.