Depending on how much you waste.
People buy by big bales from farms and waste a lot and it is still very cheap. Honestly I had very mixed experience with buying big bales. If you live in an apartment you need to split it to a few bags/boxes and also I had some really bad experience with the insects in it and 3 bales were just bad old and stinky inside, dusty, wasn't possible to feed my rabbits so they were cheap but when you calculate I've lost my money for 3x big bales and had horrible experience with insects, my rabbits got mites so cost me lots of money, time and hassle to get rid of them and cure all the bald patches on their fur etc.
Never again.
Maybe if you live in a country house and have lots of room to store it, and time to take it by small sections, shake it outside to remove all the dust and maybe steam it or freeze to make sure there's no mites or bugs, it is really much cheaper, but you invest your time and effort too.
Also a few times I've got really good hay buying by 20kg (40lbs) bales, very very good, but was maybe 3 times, then it was very mixed as I said.
I also tried different haybox stores and was sometimes okay sometimes not very good. In fact when you buy 5kg box it is not very cheap and when you buy 15 kg box it gets cheaper but it wasn't easy to manage, very messy also even from the same seller it was first time excellent and once it had insects and quality wasn't great.
Now back to the reputable brands with dust extracted they also sell by 2,5kg (5lbs) or jumbo packs 4kg so not bad and I know that hay was steamed and has no insects. Mites can be very hard to get rid of.
More expensive but you can actually train your rabbits not to waste a lot, when I first started toilet training etc I had hay in their toilet boxes as it is usually suggested and hay feeder over toilet box as well, which leads to lots of wasted hay, especially some rabbits just like to pull it all from feeder and pee on it and then they won't eat it and it all gets really stinky even over wood pellets so you have to clean and clean and clean and waste and waste and waste. Now I use different setups and tactics and there's very little of wasted hay thankfully, so you can also rethink your setup and depending on your rabbits and their personalities, their habits you can make less waste as well.
So if you got 5lbs (roughly 2,5kg) it depends how much they will eat and how much they will just soil and you will have to waste. If you manage to make them eat most of it with minimum wasted hay it can be good for 3-4 weeks (also if it's good quality hay and there's no dust), some hays my rabbits pick up every little piece of it, and some hays they leave a half of it. Thankfully they don't pee on hay anymore (except of Bernie and Harley) so it is not stinky at least but still no use.
Some hays are so tasty they eat them faster than others.
So it all depends.
You can also steam farm hay to make sure there's no mites but not always easy tbh.
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