I spot clean once a week-so remove droppings and any wet patches.
I havent actually ever done a complete full clean. Every month I take out half the substrate and replace it with fresh, mixing the old in with the new. It helps to reduce stress on the hamster.
It helps having a large cage so the mess isn't all over the cage, just in one or two corners.
I re-arrange the cage layout every 2-3 weeks though to give my hamsters variety