The short answer to your question is "the vast majority", and that this is the continuation of a trend that's been accelerating pretty much since the dawn of cinema.

Lets look at a case example - comparing the Top grossing Summer action films of 1983 (Return of the Jedi), 1993 (Jurassic Park), 2003 (Pirates of the Caribbean), and 2013 (Iron Man 3). If we take the North American domestic numbers from Box Office Mojo, then adjust them for inflation relative to 2013 that should give us a "close enough" apples-to-apples comparison.

So here's the percentage of the adjusted total domestic gross that each film had made by your 60 day mark:

Return of The Jedi (1983) - 72%
Jurassic Park (1993) - 80%
Pirates of the Caribbean (2003) - 91%
Iron Man 3 (2013) - 99%

There's absolutely a clear trend (and it keeps going back as far as you want to look) of films "earning out" faster and faster in theaters. Let's take a look at the average number of screens each film played on over it's first eight weeks:

Return of the Jedi (1983) - 1,114
Jurassic Park (1993) - 2,420
Pirates of the Caribbean (2003) - 2,856
Iron Man 3 (2013) - 2,974

Over it's first eight weeks "Iron Man" played on almost 267% the number of screens as "Jedi" - so of course it made it's money faster. So one obvious reason for the acceleration in earnings is that there are more screens available to audiences over time - the people who want to see a film have the ability to do it faster. For comparison E.T. was in theaters for over a year, when it came out because the number of people who wanted to see the film, far outstripped the available number of theaters. Lack of home video is also a contributing factor here, but muddles the issue somewhat, so let's ignore it for now.

Earning out faster certainly doesn't hurt the studios (in fact it lets them concentrate their advertising budgets much more effectively to get more bang for their buck). But what about your second question - what has this done to the model for theatrical exhibition? Let's look at an eight week per screen average for our films:

Return of the Jedi (1983) - $50,525
Jurassic Park (1993) - $28,563
Pirates of the Caribbean (2003) - $12,238
Iron Man 3 (2013) - $13,006

Sure enough, with the rise in the number of theaters, each one is making a lot less per film, per week (I don't think it's a coincidence that "Pirates" and "Iron Man" are a lot closer given how few new screens were built between 2003 and 2013). It's especially important to remember with these that Hollywood always reports gross numbers only because it's good marketing - that's not what anyone is *making* - the gross has to be split between many parties, the studio, potentially a service distributor, and the exhibitors themselves.

In terms of the exhibition model, this shift to faster earnings marks a major "sea-change" difference. Not just because each film is bringing in less money, but also how the accelerated speed affects long term viability. I did a quick napkin calculation as to how long each film could stay on a screen if the theater was *solely* dependent on ticket revenue to cover the operating costs of the screen (staff, lights, utility, rent...). How long could a fictional "average" theatre pay the bills only using their share of ticket revenue?

(Yes, I had to make some big assumptions, but I suspect I'm probably at least in the right ballpark)

"Jedi" would have *still* covered operating costs for our fictitious theater after 22 weeks when it's run stopped - not because it wasn't making significant money, but because there were even *more profitable* films available to switch to (if you ever invent a time machine, the early 80s would have been an *amazing* time to own a theater chain).

"Jurassic Park" would have only been viable for 16 weeks making the same assumptions.

"Iron Man" and "Pirates" on the other hand, would have been losing the theater money after only 9-10 weeks each.

Again - these are for the most successful films of the entire year.

So the next time you're heading off to your local multiplex and wondering why there's *such* an increasing emphasis on concession sales, on-screen advertising, VIP experiences, Live Events, space rental, kids birthday parties, or other aspects of their business compared to the past, there's your answer: Selling tickets, even to the most successful movie of the year, wouldn't likely come close to covering the costs of running a theater as an ongoing concern. So the business they're in isn't (or isn't exclusively) selling movie tickets to consumers.