It really depends on how much your rabbit likes hay. Some rabbits love it and eat a lot and others just pick at it or don't like it at all. I'm not that great with metric measurements, but I think it might last anywhere from a week to a month.
Bunaluns should have unlimited hay and all rabbits eat different amounts. Grazing is important because it gives them the nutrients they need and also helps to keep their teeth inline and prevents them from curling (which can be painful and prevent your rabbit from eating at all). Based on how much hay Sweetie's rabbits chomp I would say that: roughly 2x 425g balls of hay will last you one month.
Hope this helps