Generally, a single-slot class was 45 minutes. So a slot at "10 AM" would have started at 10:15 AM and ended at 11:00 AM.

Most lectures were allocated double slots, though, for example from 10 to 12. In that case the actual official lecture time would have been either 10:15 to 11:00, followed by a quarter hour break, and another 45 minutes from 11:15 to 12:00. Alternatively -- and probably more commonly -- there was no break, and the lecture was 1.5 hours from 10:15 to 11:45.

At the universities I've been to classes were almost always 1½ hours, so until 11:45am.