The best answer is when it starts to smell. The standard answer depends on the rabbit, it's bedding, if it's litter trained or not, and the size of the cage. If it's a good bedding, spot cleaning daily (picking out pee-spots and clusters of poo) will help, with a weekly cleaning. If it's litter trained, clean the litter tray daily and the rest of the cage every 6-10 days. A larger cage will require less cleaning.