I’m not sure about rabbits but I know about horses. And rabbits are very similar to horses. An active horse can pretty much eat as much hay as they want. My mare usually has access to unlimited tiomthy/clover hay. There is not much calories/carbs, etc in hay, not much nutrition either, it’s basically just roughage. If the horses is lazy and out of work the hay should be restricted because they can gain weight from eating all day.. they can get a “hay belly”. Horses are like rabbits and there stomach should be working food often during the day. The more often they are fed (in smaller quantities rather than a couple of large feedings) the better.
Don’t know if it’s the same with rabbits but I assume it’s pretty similar. My rabbits get a large handful twice a day and some days if I notice they have none in the afternoon I will give them a refill. I like feeding lots of hay because it keeps them busy too.