Your maximum usage, in the hottest summer months, will be about 3 ppm/day, when water temps dip below 60 degrees, it drops to zero, other months, in purgatory, could be figured at about the average of the two. For you, I'd guess at about 90 gallons, absolute max.... for the first buy, and then adjust as necessary. (Note: that is a high figure, because I didn't calculate a closing. Deduct for closing the pool.)
If you are contemplating a seasonal buy, the LAST thing you want to do is take delivery of [X] amount of cases of bleach. It is (IMHO) smart to contract for [a lot of] bleach at the beginning of a season, payment to be agreed upon, with weekly draws against your account out of the vendor's fresh stock. Otherwise, that awesome 12.5% you bought will be about 5%, when you get around to using the last of it..... YMMV.
Buy short, the first year, unless the vendor is willing to carry over to the next season. Just don't "stock up," yourself.... and check the manufacture date on the bottles you pick up....