Based on my daughter and trials, it was about getting her on the bike(s) and getting the habit of going out often so yes mates is a good idea, if you can find some.
As for length of time I found for my daughter it was two factors, interest level and energy level. If the interest level was up I would let her play tell she was tired, if interest wasn't up I would just do a short session to keep up the habit of doing it.
On a side note I have heard that the jump to the larger bikes from the Osset's can take the fun out of it for some and being willing to switch to bicycle trials might help. Don't know this from personal experience but it might make a difference.
Best of luck!