Mine are 7, 6, 6 and nearly 2 and, as we don't have much money and DH gets home from work after 6:30pm, I began a plan two years ago that they could all do one thing each and stick to it for a year, then change to something else, and after a few years of doing that, they could pick one to stick with and get good at. We are not a very sporty family so I wanted to help them develop those skills.
For the first year, they did ball skills (called Enjoy-a-Ball, absolutely fab) but it was a bit far to go, and when the DTs had finished their session, I then had to keep two four yos and a newborn occupied at the side of the hall while DS1 had his session so it was tough but managable.
Last year, DS1 did Tae Kwon Do on a Monday night and the DTs did trampolining on a Thursday. Both activities were around tea time so it did interfere with that two nights a week (maybe I wasn't organised enough but it had a knock on effect on bedtimes, etc).
This year, I have got the three of them doing the same activity at the same time - swimming - so there's only DS4 to occupy in the meantime. In addition to this though, school have started lots more free clubs. Some are at lunchtimes which is great! DT1 does a singing club after school one night but it's only half an hour so I just sit in the car reading with the other three. And after half term, they're all doing the free drama club on the same night which means I just have to go an hour later to pick them up - bonus!
Anyway, what I am saying in a long-winded way is that I felt it was good to expose them to a few different things while keeping the sanity of the family intact. I feel that 12 months with each activity means they all get a decent chance to try them and when they are a bit older they will have an opportunity to make a well-informed decision about which one to stick with. DH isn't as happy because he really wanted DS1 to get more belts in TaeKwonDo and learn how to persist, but the way I see it is that he's taking a break for a while and learning a skill that's important to get while he's young (swimming) and then he can go back to TaeKwonDo at any age and continue from where he was.