They never stayed at school through summer. Sometimes they did for the Christmas and Easter holidays.
I'm not sure, really. My perception of magic has always been that it doesn't always just do things for you, but assists. So I think she still has to put her back into the cleaning, managing the garden, etc., so to speak.
I imagine she cleans, gardens, cooks three huge meals a day even if it's just her and Arthur, bakes her own bread, makes her own butter, bakes cakes, knits, listens to the wireless, writes letters, visits friends (taking along aforementioned baked goods), tends the chickens, shops on Diagon Alley, manages the finances.
Although to be honest, I think that when J.K. Rowling wrote her character she just wrote a stereotypical stay-at-home mother who raised all the children, and didn't even think about her going to work even though they were poor. It was created in the nineties, and such issues weren't as discussed as they are now. Her not working matches other old-fashioned aspects of the books, I think.