Technically, 4oz is supposed to be allowed. The rules published on TSA.gov allow 4 oz (which approx. = 120ml) of nonprescription medical items, specifically stating saline as one such item. Many idiots amongst the ranks are claiming that contact solution and saline aren't the same things for those purposes (most, I guess, are too young to remember when pure saline was indeed what you used for contacts, before all the multi-purpose solutions with enzymatic cleansers, disinfecting agents, etc. came along).

I carry a 2.5 oz bottle of solution with me that came in a sample pack at the doctor's office, which is sufficient for a week's trip for me, but it irks me that I would either have to go back to the office to get more or else refill that bottle repeatedly as I travel (which can be unsanitary and could lead to vision loss )