Rests depend on distance run and intensity.
Usually for sprints, the cardinal rule is about 1 min per 10m run. However, this may not apply to longer sprints, but for you this would probably be fine.
Ext. Tempo can range from 30 sec. to however long (like 5 min) depending on the distance run. Give an example and I can give you a good rest period.
Int. Tempo is usually 1-5 min depending on distance and again an ex. would be nice.