illimited. But as every new food, the first days you should give them only a little amount... but this is what you should do even if you decide to give them a bell pepper for the first time.
Once you are sure their gut digests the grass normally they can eat whatever they like; grass does not contain sugars and piggies don't become fat.
When I adopted my two piggies and they were young, they came home already used to the fresh grass...
Check the poo and when you cuddle your piggie, put your ear in contact with a side (for 30seconds and more) and then with the other side. You should not hear any "gurgle" (only the heartbeat)