I offer 6 weekly anyway to start with.
Larger domestic jobs (£40+) I wll do 12 weekly if they ask (I don't suggest it though). However, I make it clear that 6 weeks may mean 6 - 8 weeks if I struggle to get around such as in a freeze-up or if I'm on holiday etc.
If they want it less frequently than 12 weeks, I normally decline. However, if it's a large, lucrative job, then I do think of it as a not too bad first clean each time and price accordingly. 16 weekly or less and it's probably going to cost them 50% more per clean. If the 16+ weeks has been caused by something that isn't down to them (e.g. a 12 weekly where I've been slow getting round) then naturally I don't charge the extra.