I'd grab at least 1 month. You'll get the full rules through Wargames Vault, which means they'll be in your digital Library there and you will benefit from any future updates to the pdfs no matter your Patreon status. But I've not found this part very smooth as they only send out Wargames Vault codes on one particular day of the month and you could end up paying for a second (calendar) month waiting, and I don't remember which day it is (best sub on the 1st and drop it on the 30th I guess)
Apart from that, I sometimes sub for 1 month just to use the unit builder and stat up a whole new faction, export that to pdf, then drop the sub