I did it for a while quite a few years ago. Late 80's early 90's. I quite enjoyed it. I believe that commitment was about 3 hours a week. You could split it up if you wanted or do more if everyone was agreeable. They had some special events to go to like free days at the zoo kind of things. The did say it is about your time and try not to spend lots of money on them all they time. It doesn't require you to always do things that cost money. When I was with my ex we were both big brother and sister to a set of twins (both boys) and I just loved them and we had lots of fun. But unfortunately their mother kind of soured my experience by cheating the system. Had husband living there while supposed to be gone, bought themselves hugely expensive gifts at Christmas (diamond rings and professional drum sets) and signed the family up for all the christmas hampers and would hide the other hampers as they were being dropped off. The stuff covered the whole living room and she was proud of herself. Afterwords, I had another little boy who was cute, it was sad and he needed lots of reassurances and would call constantly and crying that he wanted me to come and get him. Sometimes I just couldn't and it would break my heart. They eventually moved away.
It can be extremely rewarding and lots of good experiences but it can also be a little hard on the heart. But I don't regret doing it. After that last little guy I needed a break and with work it was really hard to find enough time. I have thought from time to time doing it again.
Also, there is another program where you would take out a special needs child (you can determine what you feel you can handle), it gives the parents a break and it allows the special needs child to experience things the parents may not be able to do with their child. Some parents are too scared, overwhelmed or embarrassed to do some things with their child or also it allows the parent to give another child some one on one attention while the special needs child is out. You actually get paid to do it and they cover the costs of whatever you do, whether it's a movie, dinner, camping, etc. A friend of mine started doing this many years ago with a down's syndrome boy and they are still friends today over 20 years later. He was like a member of their family. She even took him to Costa Rica with her a few times. If you want you can PM me anytime if you have more questions.