I only have 3 kids, well, 2 of them are adults now, but I change the sheets usually fortnightly or close to it, depending on the weather, if we happen to get a good drying day then I will quickly whip all the sheets off all the beds, but I don't really have a set schedule for it. Seriously, in the grand scheme of things, what are your kids going to remember about their childhood, that you changed the sheets every week on time or that you spent quality time with them and that you did your best? I used to let these things worry me all the time, about not getting things done, but honestly my kids don't even notice half the time if it hasn't been done! So now I just enjoy what I can and get whatever I can done, there is always tomorrow!