It depends on seniority, base, and the way you bid. As a line holder you can get anywhere between 1-5 legs per day with anywhere between 30 minutes and 3 hours between arrival/departure. You can be on duty anywhere between 2 hours and your max (max depends on number of legs and when you start your day day).

As a junior reservist, you'll probably sit a lot, and have a lot of deadheads. Most of my 4-days have been about 17-20 hours. Some days are busy, some days possibly only 1 leg and could include a 30 hour overnight. So you see there are a lot of factors, and no two days are usually the same.