Seriously, unless you have a unusual reason for suspecting the tank is poorly built or damaged, there really isn't any reason for water testing for more than a day or two IMHO.
Yeah, overnight should be fine. If you do it for too long, like two of the smarties suggested, you may end up with a leak eventually since most tank are not meant to last forever.
I built this 37 from scratch. I filled it with water and let a 700gpm pump circulate for a week. Maybe overkill, but I had never built a tank before.[/img]