I assume that your question refers to travelling to the USA WITHOUT a visa (under the Visa Waiver Program, with an ESTA), since the '90 day rule' only applies to such travel.
ESTAs are normally valid for two years (or until the expiry of your passport, whichever is the earlier). They are valid for multiple entries into the USA (of up to 90 days at a time). There is no minimum period for you to be outside of the USA between each visit. However, neither an ESTA nor a visa actually guarantees you admission into the USA. Immigration officials retain the right to refuse you entry. If you were blatantly trying to cheat the system (e.g. by taking a couple of paces over the Canadian border and then turning straight round) you might well find that you were refused entry.
Chris