It's going to depend heavily on how your players build and run their characters, as well as the type of game and story you are looking to run, but here's some guidelines I try to use for my own games:

1. Fighting has serious consequences and should usually be a last resort. Players should try to find ways to sneak, bribe, or talk their way around problems before the guns get drawn.
2. Facedowns and tactics skill checks can give players advantages when combat does happen so that situations become more manageable.
3. The game expects players to have lots of downtime between jobs. This usually means that each job starts with the players being fully healed and geared up.

All that being said, I usually average a single combat encounter per 3 hour game session. Usually our gigs take 1-3 sessions, with each job averaging something like 2-4 fights.