depends on how heavy the loads'll be that you're placing on the floor AND what the base material's like,,, my garage floor's 5" thick & has 8" graded aggregate base compacted in 2 4" lifts over ga clay,,, think some call gab crusher run, item 4, or even ' mansand ' :laughing:
if you're filling 8' between the ' concreate ' walls, be a good idea to go no more'n 8" lifts.