used to mean that time passes surprisingly quickly
Time flies when you’re with friends having great time.
time ˈflies idiom
(saying) time seems to pass very quickly
• How time flies! I've got to go now.
• Time has flown since the holiday began. This phrase is a translation of the Latin ‘tempus fugit’.
Cambridge Advanced Learner's Dictionary - 4th Edition
time flies saying
C2 used to mean that time passes surprisingly quickly:
Time flies when you're having fun.