How long am I going to be away from my wife and kids when I’m in the field as 12B active duty?

It is going to depend on the unit you end up in and their operational tempo. If you go Airborne and land at the 82nd, expect to go on multiple field exercises a year and be prepared for a deployment. The overseas deployments have slowed down of the last four years, but expect at least one during your enlistment. If you land in a Mech Unit, you are still going to the field, although probably not as much as the 82nd or 101st. However, chances are you will catch at least one NTC rotation and that ends up being about 30 days.
The price of financial security for your family is being away from them for periods of time. I got married on a Friday and left for OCS the following week. Even in the National Guard, I probably averaged 60 days a year or more away from home.

Blunt reality is you need your family to understand everything you get from a paycheck to housing to medical care is from the ARMY. Army comes first and the sooner your family understands this, the better the relationship will be. Looking for concrete answers how long you will be out will only set you and the family up for hardship. My wife married into the Army and as a recent immigrant that could not drive with a new baby I left her for two weeks with my parents. It was rough as hell for her but I started from day one this is Army life. Eleven years later she and my children understand it very well.

Could be hours. Could be days. Could be weeks or months. It will all depend on the training being conducted.