How long would you suggest a 12 year old to meditate for? And when?

Depends on the 12 year old and what meditation is being taught. For a mentally and physically healthy 12 year old I would suggest teaching moving meditation. As with any practice start small, like 5 minutes and add a minute when that time gets comfortable.

I would start small maybe 5-10 mins to begin with. To me the time would depend on them. If school is an issue, maybe first thing in the morning to get the nerves out. If winding down for bed is an issue then that might be the best time. But there's no reason why both times wouldn't work either. A time to think about the day & then time to reflect on the day. I do both personally.
Hope that helps…