I've been a cub scout leader and a girl scout leader (mostly older girls).
The funniest joke ever told is "It's only an hour a week!" Yeah, right. But I will say it's VERY rewarding.
My personal assessment of the time involved.
Meeting: 1 1/2 hours per week (This was for the girls 6th grade and older. And 1/2 hour was spent working with the younger girls we were partnered up with).
Service unit meeting 1 1/2 hour per month.
Paperwork averaged 1/2 hour per week, 2 hours a month.
Planning: Again, with an older troop, the girls did some of this. But based on my cub scout experience which was closer to your girls' ages, I took about 4-5 hours the beginning of the year to lay out a plan, then about an hour a month to be sure I had everything I needed. That was just for the meeting activities.
Non-meeting activities: This includes outings, campouts, thinking day, etc. These kinds of things tend to bunch up, but on average, if you have an active troop, you're going to spend at least one extra day a month on some kind of activity. As they get older, it becomes 2 days when they camp, and I tried to camp at least 3 times a year with my girls. Planning for those extra activities usually averaged 3 hours a month.
Training: Boy this one was varied, and was my main gripe with GSA. Just to be allowed to "officially" lead your girls, you needed 4 hours of "basic" training, and 4 hours of level specific (i.e. brownie, junior, etc.) training. Then if you want to camp with them, you need 1. camping, 2. outdoor cooking, and 3. First aid (unless another adult is certified). Each of those is a FULL weekend through my former council! So you've now given up 4 weekends that you could have been spending with your family or your scouts. And OBTW, you have to re-certify every two years, regardless of how often you scout camp! Luckily, my co-leader was camp certified, and I did the Red Cross first aid thing (one full day any way you slice it). Now I've been camping my entire adult life! I tried to get an option where experienced campers could do an overnight where they simply demonstrated their skills, but that wasn't possible. For Cub Scouts, that's what I did, but GSA wouldn't do it.
So throwing out the training requirement, I probably spent about 4 hours a week on scouting stuff. Some weeks it truly was just the 1 1/2 hours of meeting, but others it would end up being almost every non-work waking hour.
I did know what I was getting into, since I'd been the "responsible adult" for DDs troop long before I led it. And I finished my Cub Scout committment before taking on the GS troop.
I had to leave my girls behind when we moved here. DD hasn't wanted to re-join. She said it will just make her miss her friends from Md scouting. I can honestly say that I hear from all of those girls on a regular basis, and they all say how much they miss me. They ended up merged into a larger troop, but they don't do nearly as much.