If you mean how soon you after coming home you can return to the USA for a second visit - there is no set answer. It's entirely at the discretion of the US immigration authorities.
A second 90 day stay immediately after a first one might raise eyebrows - a third would almost certainly raise the suspicion you were not a bona fide tourist.
A rule of thumb I've seen quoted is that you should be out of the US for as long as the visit - but there is no official backing for this and it's not a rigid rule.