In function create on model A, you will check max time of user and compare with new time value.

  1. You get max time in model B by user and convert it to float ( ex: 9:30 = 9.5 ).

 2. Compare max time and new time.

   - If "new time" - ( 15/60.0 ) > max time"  ( 15 min). you will create new record into model B ( or update max record if you need ).

  - Else you update max time  in model B. 

I hope my comment will help you for this issue.