It will be at least 150GB, which is the current cap for Max. 150GB is the high end of the range AT&T declared for its trial. I think Max 18 may have thrown the people who decided on the allowances off guard a bit. It's still missing from the AT&T usage tool FAQs, even though there are potentially customers on Max 18 in the test market.
I would guess 200GB, but 250GB might be a smarter choice to placate people who want AT&T's offering to measure up to the caps Comcast has set for its customers. -- AT&T U-Hearse Your funeral. Delivered.