There are lots of clays on the market now but the megs kit is good. Bilthamber do 2 different clays hard and soft depending on the contaminants on the car which are very good. If the sun is beaming and its hot i would do a panel at a time , once done , dry and move to the next panel . As for how long it takes it all depends on how bad the surface is and of course how big the car is and how fussy you are . Its taken me an afternoon before on a bad example .
Its worth mentioning that claying can marr the paint by the nature of it, keep it lubed well at all times and even then i,ve seen marring which could need polishing out.If you drop it on the floor my advise would be to throw it away as little grains of dirt will mark your car eventually .

hth