"frontend" modules (i.e ones with the default autoscaling, and thus instance classes named starting with F) all share the 28 hours free quota (which means 28 hours of F1, remember: if you use a default F2, twice as powerful, that's 14 wall-clock hours per day).
"backend" modules (i.e ones with manual or basic scaling and thus instance classes named starting with B) share a separate 8 hours free quota (again: that means 8 hours of B1).
As you give us zero information about scaling kind and instance classes you're using, that about exhaust the information WE can offer!-)
Alex