I have a 5½ month old mini schnauzer..how much exercise
should he be getting?

At the moment, he gets a 15 minute walk every morning, lunchtime
and evening, plus lots of playing at home in the evening i.e. ball games etc.
We also take him to the park a few times a week, or to the beach at the weekend.
He also plays with the two schnauzers next door, and is starting puppy
classes next week.
Is he doing too much? He does not often get tired (only following car journeys
really).

He is getting plenty of mental stimulation too, as he doing very well
with our regular home training i.e stay, sit etc.

I am aware too much is very damaging....can anyone help?

That sounds fine, Louise...it's not as if he's a big breed with masses of growing to do, and a (comparative) lot of weight on undeveloped bones and joints. As long as he's not exhausted after every walk (out of breath for a short while is fine) it sounds okay to me. :)