As an infantryman, how long and often can I expect to be in the field? Is it dependent on the unit I'm with, or same across the board?

Unit dependent, but even in the best, or worst, depending on your point of view, you will be spending considerable time in the field. We train to fight and there is seldom a battlefield in cantonment. Sit down and go over this with your wife and make sure she can handle the family bills and things while you are in the field. Often times the units enlisted wives have some kind of a club that offers both help and companionship when you are in the field or deployed.
I got married on a Saturday and left the next Friday for OCS, which was kind of the lifestyle for the next couple of years. Even as a National Guard Engineer, I average more than 70+ days a year away from home.
Oh, and I am still married to the same woman.

All of this will vary by unit, and assignments!

It really depends on the unit and you(believe it or not!) You have unit deployments, and then you have a variety of schools that you can attend. One year I was home three months out of 12, and not all together either.

Unit's mission dictates training , so it's never the same across the board . Some unit's have 9 month's out of the year in the field while others 3 month's , with the average being in the middle , type of unit also affects field time , Mech Infantry tends to do more field training they carry there own support with them , vs Airborne or Air Assault, Mountain troops also spend above average time in the field , but regardless the specifics of your Infantry and your not training in the rain and mud , your training like Cavalry , roflol