Well this is a tricky question and I have no one single answer.
When I was owned by city kennels or slaving houses I expected no time. Perhaps a scene or two to get me into the house, but that's about it. I probably went to far at times when I'd get an IM from the slaver wondering what I've been up to and how come I was never in the kennel LOL but unless I needed them to forward a scene for me I was fine on my own.
For a private master or collar my expecations are a bit different, I would say that of any RP partner in any genre, but it's particularly important if you play a slave due to the limitations of the genre. I don't expect them to be online all the time, nor expect them to spend all their time with me especially if they have a business, sim stuff or OOC things to attend to. However, I'd be lying if I didn't say that spending a good portion of their RP'ing availability time together, developing our characters and our storyline wasn't important. Doesn't have to be one on one time, and as Kaitlin said I don't mind being window dressing or background, but time together is important to me.
I have refused collars and asked release under these circumstances in the past, due to lack of RP time. Not because I expect 100% attention all the time, but because I never want to be an obligation or burden. If someone is just too busy, torn in too many directions to really have an RP partner, it's best to go our own way rather than let any bitterness develop. It doesn't mean you can't still RP together, certainly doesn't mean you can't still be friends, but that expectation of a joint storyline isn't there. I think that's part of why I enjoyed being a coin girl so much. People would show up to play, it's not like I was hard to find working the docks, and if I saw them again the next day, the next week, or never it was still a good time with no pressure on either side.