How many feet do you need on the ground for it to be ruled a complete pass if forced out of bounds in the NFL?

If a receiver is knocked out of bounds by a defensive player and the official believes the receiver would have come down in bounds with both feet had he not been contacted by the defender, then it's ruled a completed pass. ---- This rule has been changed for the 2008 season. There is no longer a 'force out' rule in the NFL.