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.
Right now during recruit training,
It takes officially 1 night a week (Wed) 2 weekends a month and 2 - 2 week courses, we had one in jan and the next one is in July.
However in actual fact, most of us train for at least a couple hours every night, plus Hill walking in our weekends off, not to mention prepping kit and fixing bits here and there.
It all adds up to a pretty big commitment especially if you are working full time as well, don't expect to keep much of a social life outside the unit.
Depending on which RMR unit you join will depend on how high the commitment level required is. While all require the 1 evening a week, two weekends a month and the 2 fortnight training courses, RMR Bristol can hold you on phase 1 training if you take longer to acheive the standard. While another RMR unit requires you to acheive the phase 1 standard at the set time or come back and try again next year.