I've always taken care about my guests, even if they were volunteers. There is a lot work to do here, but basically the help I need depends day by day: sometimes I need a huge help (4-5 hrs), sometimes you just have to help cleaning (2 hrs) and then you can enjoy your day left in London. Basically I don't care about how many hours you work, I just care of the final result. It depends on you: I tell you what you have to do during the day and then it's up to you how many hours you need to finish it (obviously the maximum is 5 per day).
My first thought isn't to start working as soon as possible: I've always given the time the volunteers need to arrange their stuff, and then gradually I start asking help around the house. We are a big family that loves sharing our stories and cultures. You're more than welcome to share yours with us!