I don't know how many you're going to be giving it to, but if it's several, you'll be surprised how fast it goes. I've been giving it to 6 this last while, and an 8 lb bag lasted me less than 3 weeks. The last time I bought a 16 lb bag, and it's going down fast.
If you keep the BOSS in a cool dry place it will be okay for at least a few weeks. Normally I would have said that a bag of BOSS would keep a long time, no problem, but this last bag I bought sat in my truck from when I bought it to when I got home at 4:30, then I took it out & set it on a hay bale just outside the hayshed, in the sun. It wasn't a real hot day. When we went to do chores an hour or two later, the bag was sweating really badly! I took it inside & opened it & moved half the seeds into another bag, and then stirred them all up a few times, just to make sure they were dry and cool. They weren't actually heating yet, but soon would have been--when I stuck my hand into the middle of the seeds they were starting to feel warm. I guess I didn't think about how much moisture there is in the seeds! I think it says on the bag that once it's opened it should be used up within....30 days? 6 weeks? I'd have to look again. In any case, if you're feeding several horses I think you'll find that the BOSS is used up within a few weeks.