For years, here’s how things worked in Apple’s app store. You discovered a mobile app you liked, and then you either downloaded it for free or paid a small one-time fee to purchase it.

But a new subscription service launched today for the iPad by Seattle’s Big Fish Games could turn that model on its head. The service, dubbed Play Instantly, allows casual gamers to access dozens of titles for free for 30 minutes per day and then subscribe for $4.99 per month if they want to continue play beyond the allotted time limit.

The advantage of the service is that game players can experiment with a variety of titles, playing each one for a limited amount of time without a separate installation. Big Fish is hoping that folks will get so hooked on certain titles, they’ll upgrade to the monthly service.

That model has worked well for Big Fish in the past on its own Web site, but on other platforms it is a bit of uncharted waters.

Big Fish calls it the first subscription service for games in Apple’s app store, with vice president of business development Will O’Brien saying that it “will allow frictionless and immediate access” to the company’s game titles.

At this time, the service is only available for those connected to wi-fi on an iPad. But Big Fish plans to expand the subscription offering to new platforms next year.

Will a subscription service actually work for games? The company is already seeing strong demand, writing in the app store that the service is not currently available due to high demand from users. (That’s not sitting well with some users, with one fan of Big Fish writing in a review: “Why offer this at all if no one can actually get in?”)

Big Fish, which raised $84 million in 2008, posted $140 million in revenue last year and has been discussed as a possible IPO candidate. Paul Thelen, the former RealNetworks employee who founded the company in 2002, tells Bloomberg that Big Fish is in a position to go public.

“We’re at scale, have great momentum and remain in a position to pursue a public offering or any number of alternatives if the markets allow,” Thelen told Bloomberg.

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