Dont do it. Years ago I had one on working livery, he was overworked, I had to book him for when I wanted to ride him and still got charged for feed,shoes and vet and because several differant riders on him at didn't levels of riding, he got sour and started napping, took a year for him go back to lovely natured horse that he was. I went to a DIY yard, before I bought my own place.
My horse is on working livery at a riding school - I think if you choose the right place it can work really well. Ours get all year turnout with 24/7 in the summer and I get reduced shoeing £67 down to £48 as he does the whole yard (24 horses) . Ours always get friday off and generally another during the week and very rarely work for more than two hours a day - with my horse it is normally one as he an intermediate horse so there are fewer people riding at the standard he is . I work a couple of really long days a week so I ask for him to be put on lessons on these day so that I know he has been exercised (he is a tb and needs lots of work) . They do a mixture of schooling,hacking and jumping so they are not going round and round in the school for hours. We have an indoor and an outdoor school and we are not allowed to use them during lesson time but they are free to use during any other times . I have been at my yard for nearly 20 years now and it does work for me and if anything has helped desensitise my tb to noise,dogs,kids etc as he had led a bit of a sheltered life as he came out of racing .
Mine is on working livery, has been since november last year and it was all going well until he started headshaking 2 weeks ago and they don't know why. Vet doesn't know why either . But other than that I had access to him whenever I wanted, as long as they got him 12 hours a week. He was never used those 12 hours tho, and most of his lessons were slowish ones, so he was never worked hard for an hour straight. He seemed to be enjoying it.