Winning times for USOF are spelled out in the competition "
Rules" found in Robin S's Virtual Binder.
Specific rule regarding length of courses is found in section 24, specifically,
here. I'm not sure whether sprints are covered (yet) in the rules; I didn't go looking, but they may be addressed elsewhere. They're a relatively recently adopted format, thanks to promotion by PG and members of the US Team.
USOF rules always seem to be a work in progress; anyone willing to spend time on a committee reviewing and suggesting revisions to them would be welcome, I'm sure.