Have you been keeping up with their weekly weight checks? That is your guide as to how much they are eating.
Eating grass is absolutely fine for them to do (it works on their teeth etc in the same way hay does), but its important to build their time and access to grass up slowly so you don’t cause digestive upsets.
If their weights are fine, grass has been built up slowly and not causing digestive problems, then they should be ok.
Mine spend around 14 hours a day on grass and I make sure there is always trays of hay available in their lawn run as well - they go between eating hay and grazing grass throughout their time on the lawn