For / since / when / how long

for and since
for + how long
  • for twenty minutes
  • for three days
  • for six months
  • for years
since + when it began
  • since Monday
  • since April
  • since 2019
  • since we arrived
Examples
Sally has worked here for six months.
Sally has worked here since April.
I haven't seen Tom since Monday.
Watch out
✗I've been waiting since two hours.
✓I've been waiting for two hours.

Two hours is a length, not a starting point. Use since two o'clock instead.