Hay is the main part of their diet. There is no set amount that they should be eating but they need to be eating enough hay to maintain their weight.
We use kg in the UK so I’ll talk about both measurements
Your usage converts to
18kg in 8 weeks (40lbs)
2.25kg or 4lbs per week for two piggies.
I can’t do a completely accurate calculation as I have five animals eating hay but trying to calculate it down just for my pair of piggies works out to approximately 1.6kg or 3.5lbs per week. (Actually in reality it’s probably a little more than that when certain bedding factors are taken into account across all my animals)
I buy 20kg, or 44lbs, which lasts all my animals four weeks
However, it’s not exact for anybody and never will be - there are going to be variations in set up and some will use more hay than others due to the way they serve it.
Those lovely large piles of hay you give provide enrichment for the piggies as they can forage freely, run through, sleep in the hay etc. It may feel like you are ‘wasting’ it and going through a lot but those large piles of hay are so much more than just food so try not to think of it as wasted hay - it’s been completely used for fun and enrichment and doing what piggies want to do in big piles of hay.
My own two piggies live in my shed and due to that I use lots of hay as their whole hutch is filled with it, and a pile of it is also put out in the shed floor for them. It gets spread around, topped up daily, pooped on etc and yes most of what I give them will be thrown away but that’s ok, they’ve enjoyed having It