I can't remember the last time I cleaned my fridge out, other than just before moving house.... I think my cleaner occasionally does it, usually when we're on holiday and there's not so much stuff to take out.
If something has gone mouldy in the salad drawer then I take that out and give it a good clean but otherwise it's just not something I think about.
We clean the fridge when it breaks down. I'll generally clear it out and switch it off if we are going away for more than a week or two. I clean if something has gone mouldy or spilled. But I don't clean it out on a regular basis.