You can't just throw out a random guess like "it will last at least 5 years from now" or "somewhere between 5 and 10 years".
Because it's actually quite simple: as long as developing Minecraft is a sound business for Mojang, they will continue doing it. It is by far their biggest cash cow and the very foundation of the entire company (which has grown to a decent size by now). When the turnover is no longer enough to support the company, they will have to either rethink the business model (the pay once, get all updates for free is good for us players, but not for them) or maybe sell out, but I doubt Notch would accept that as a viable outcome. Even though he doesn't actively work on Minecraft anymore, it's still his baby. I don't think he would even consider "giving away" his master piece to another company.
My best guess is that whenever they run into that wall, Mojang will try everything they can to keep it going. That could be anything; lay off staff and cut expenses in general, take on investors, have a global donation campaign and so on. Whenever the time comes when the game can't support the company any longer and they believe they have done everything they can to save it, they will make the game open source. There's no doubt about that. Open sourcing it is totally in the spirit of Notch and the company itself. So in that sense, Minecraft will never "die". But at some point Minecraft will no longer be the big thing it is today. Other games will come and steal away players. At least I hope so. As much as I love Minecraft, I trust that games will evolve even further in the future.
But when? That entirely depends on our willingness to buy new licenses.
