This really depends how many pets you have and their size. The environment also plays a part here. For example: do your pets spend their time in the city vs. living in the country? Do you do a lot of outdoor hiking or adventures with your pets? Generally speaking, a bottle lasts two to six months.
For example, I have two medium sized dogs (about 50lbs each). I live in a suburban neighborhood, and I take my dogs camping in the summer. One bottle lasts about two months.