One evening a week, one weekend a month, one fortnight a year, running in your own time about every other day.
The first 18 months or so, while you're in recruit training, are worst because you can only really miss one or two of those weekends or you end up getting back trooped and then training takes 2 years+ and you don't want to get in to that game. Later you have a bit more flexibility, provided you make up the total number of days.
It's quite a lot of commitment and guys do lose girlfriends over it. They're in the minority though, most people manage to balance it.
Sorry if that's not the comforting answer you were hoping for but nothing worthwhile is easy.