Veg is one cup per pig per day.
As long as you don’t give more than that then it doesn’t matter when in the day you give it to them: some choose to split it in half and feed twice a day, others once a day.
Mine only get their veg in the evening - they have one lettuce leaf each, a slice of pepper, some coriander (cilantro) and cucumber.
At all other times they only have hay available (I don’t feed pellets).
Are you giving them other types of veg as well? Lettuce is not nutrient dense so they do need other veggies daily to ensure they get a variety of nutrients and in particular vitamin c.
Kibble (also called pellets or nuggets) should not be constantly available to them. It is strictly one tablespoon per pig per day. Allowing constant pellets or overfeeding can lead to obesity, reduction in hay intake (and consequent gut and dental problems) and excess calcium intake (which can lead to bladder stone and sludge formation)
Long Term Balanced General And Special Needs Guinea Pig Diets