Depends on where her education was beforehand.
When to start - depends on how she is and what your facilities are.
My foal is now almost 5 weeks old. My saddler is coming out to reset Peri's saddle next Friday and will then commence the long road to getting her fit again. She will start with just 5 minutes walking and build up from there. We have our own place so it is easy to organise. I want to do gradual weaning, so it should fit in nicely with that. I'll do my walk stuff in the field, but later on Belle will go in a stable for a few minutes and gradually increase the time she is away from Peri, until by 6 months she will only have a drink morning and night and be separate the rest of the time. Then completely off altogether after that. Means that I can work Peri in the separation periods with the aim to have her almost fully fit by the time Belle is completely weaned. It also supposed to be a much less stressful method of weaning for mare & foal.
My fitness target is high level dressage so it takes quite a bit longer to get to that as opposed to hacking etc as there are a lot of muscles to build up again, too quick and I risk injury to the mare.