I go through mine the night before the trash goes out every week, to get rid of anything that the kids thought might be a leftover- pizza crusts, half eaten takeout, cheese that got shoved behind something - that kind of thing. I have 2 teens, one 'semi-adopted' friend that spends half the week with us - plus a group of 5 more that are at my house at least once a week.
The 'real' leftovers I use in meals through the week.
When I notice spills or just 'remnants?' from things (fruit/veggie leaves stems etc) I clean out the section that is affected. I keep liner paper in the crispers and meat keeper which helps alot.
About once or twice a year I go through everything, clean all the jars and bottles, take out the crispers and shelves and do a thouough cleaning.
The hardest for me is the freezer - we have an upright in the garage, but since we hunt and fish, there is rarely a time where it is empty enough to completely clean out. THAT usually happens when enough frost has built up that I can't stack things well - about once a year (maybe longer?)
When you have teens - the fridge is WAY down the list of things to keep clean - as long as I can find everything, and it looks good from the outside, I'm OK.