Rabbits need 24/7 access to hay. Greens can be fed once per day.
Some may keep their rabbits outdoors where they have 24/7 access to grass and weeds for their "greens." Grass is hay that has not been dried yet, so that is fine for rabbits accustomed to it.
In areas (like CA) where RHVD2 is present, it is risky to house rabbits outdoors. So indoor rabbits generally get different greens. These greens are fine to limit. In fact overfeeding greens can cause tummy issues with some rabbits.
Indoor rabbits need plenty of hay to keep their digestive system running at peak. Greens are secondary to hay. If too many greens are fed, they may not eat as much hay as they should. This is why limiting greens to a measured portion is generally recommended.
That measured portion can be fed once per day or spread out over 2 or 3 feedings. Whatever is convenient to make habit. Rabbits like routine, so having a set time (or times) each day for feeding greens is something a rabbit can look forward to.