score:6
Your post is confusing. There is no "90-day visa". You are probably talking about the 90-day admission period on the Visa Waiver Program. When you enter the US on the Visa Waiver Program, you are almost always admitted for 90 days (unless you have only been to Canada or Mexico or Caribbean islands since your last stay on VWP), regardless of how long you stayed on previous stays.
To enter on the Visa Waiver Program if you are arriving by air, you need an ESTA. (To enter by land, you do not need an ESTA.) The ESTA just needs to be valid on the day of entry. The ESTA is valid for 2 years from when you got it. So as long as the day you plan to enter on your next trip is before the end of the 2-year validity of the ESTA (even if on the expiration date itself), you will be admitted on the Visa Waiver Program for 90 days, enough for a 6-week visit.