That's sort of tough to quantify. You have to look at the user base and how often they use the program. So, lets try and get a good estimate of how many "free" tickets they are using:
-Lets say around a quarter of the users are very infrequent and only take advantage of it once every other month: 400k * 0.25 * 6 = 600k
-Lets say half use it about once a month: 400k * 0.5 * 12 = 2400k
-Lets say 20% use it about twice a month: 400k * 0.2 * 24 = 1920k
-And for the final 5% they see a movie every week: 400k * 0.05 * 52 = 1040k
That totals out to 5.96mil tickets in a year. Sounds like a lot, but is it? Lets estimate how many tickets are sold for all movies in a single year. In 2016 the total domestic boxoffice was about $11.2bil with an average ticket price of about $8.61 a pop, which works out to about 1.3bil tickets sold. If our 5.96mil discount tickets are accurate then that's less than one half of one percent. So for a movie that makes 200mil they'd be missing out on less than 1mil of full price tickets.
It looks like the answer, currently, is that the effect isn't all that big. But it also won't be spread equally across all films: an indie that loses out on a full price ticket will feel that effect more than a tent-pole. But then there's the other side of the equation, where would that person have gone for a full priced ticket at all? So are they actually losing anything, or are they gaining a little? That's almost impossible to quantify without really in-depth usage and polling statistics, of which I have neither. My gut says that currently these programs have very little effect on the studios bottom line.