i usually do it over a weekend, wash car with fairy liquid to get everything(old polish / wax) off. then dry and then clay. on the sunday ill then go through my normal polish (2 coats of AG SRP) then wax (2 coats AG HD WAX). youll be amazed how tiring it can be, in the summer months avoid doing it around mid day as the polish and wax are harder to use in heat, i tend to start late afternoon, in summer around 5pm.
i find the effects of wax / polish last about 2 months, i can tell when it needs re doing as rain doesnt bead as uniform on the bonnet or there are a few light scratched on the body that the polish usually hides, so if i can see it it time to get the polish & wax out. claying ive only done once, i only plan to do it 2 or 3 times a year. only way i know of to tell is if the paint feels rough to the touch it may need to be done, so far i find the best area to check is the side skirt in front of the rear wheel. when i clayed this area had a lot of muck where as most of the other panels had minimal deposits, previous owner must have looked after it im thinking.