each unit works a little bit differently than the next.....
you'll have to ask your questions when you visit your local militia unit.
The constant, for most units is that each member has been budgeted some 37 mandays between September and May - a typical training year (vs April to march budget year). Between May and August, reservists will be offered anywhere between 20 to 100 mandays of full time employment. Lots depends on your trade and the rank+ qualifications you carry.