It really depends on the rabbit. I have one who will go through most of her hay rack in a day, and it is a fairly large rack. I have others who take longer to eat a smaller amount. Some rabbits also waste more than others, but if you have a fairly enclosed rack they would waste less.
In my experience, 1 5 pound bag (when I still bought them), would last about 2-3 weeks for 2 rabbits. Once i got 3 rabbits, that dropped to about 7-10 days.
I now buy hay bales and go through about 1 bale per bunny per year. That can vary depending on the quality of the hay and the size of the bale. Bales around here are about 55-65 pounds.
For 1 rabbit, buy the largest amount you can afford and store. Smaller bags tend to cost more per pound than larger bags. Even if you were to buy 25 pounds at once, it would be cheaper then buying 5 5 pound bags or 3 10 pound bags. Since you are getting it shipped, you do need to factor the cost of that into the price. A smaller bag might end up being cheaper since it could cost less to ship. It is important to do a per pound price so it is equal no matter the size of the bag you buy.
Hay doesn't really go bad if stored properly, so you don't have to worry about it too much.