It really does depend on the class and how it's structured. For Ballet, Level IV (beginning pointe, as about age 12), I'd say no more than approximately 10 minutes at the barre at first, as a part of their regular classroom work. This is assuming that these students are coming to a ballet technique class four time a week, ideally. The exercises the students would be doing are probably going to be with two hand on the barre (facing it).
Later, as they get stronger you can progress to exercises with one hand on the barre.
Begnning center work should include basic technical vocabulary such as tendus, pliés, temps lié, port de corps, and learning how to walk in the boots -- such as "ballet walks" across the floor on the diagonal.
Then progressing onto things like élevés, relevés, piqués to two feet -- all with stays for control.
Most, if not all, of the gradual introduction of steps onto pointe should be first done at the barre. Or, even travelling down it, or in some cases, if you put two barres parallel to each other, then down both, holding on to both.
The process is progressive, and if a student is struggling too much with something, you can always go back to the barre and/or simplify it.