I often wonder how many people on this board actually play tennis, when I hear about things being a lock or so and so being unbeatable. If you pay attention, it can really be an education to listen to some of the players in their post match interviews. Any pro player, has a chance of beating any other player in the world. Obviously the chance and likelihood increases as one goes higher up the rankings, but there is a real chance there for everyone, because these guys can play tennis, well. All the players know and believe this, which is why you see immense disappointment from players when they lose, even when they are big underdogs. Players, justifiably, think and believe they have a chance of beating anyone. Federer, Nadal and the other top players also know this, which is why you see a sense of relief when they win. They know everyone out there is gunning for them and are probably going to play loose and more relaxed without the pressure of expectation from others. Look at Federer, he struggled with someone ranked in the top 500 in Dubai earlier this year. He almost went down and could have lost that match. If you look at post match interviews you will see that none of the players appear too over confident, because they know that at any time, they can be beaten. Just one slip up or average performance, and they could be out of a tournament, which is why they are so appreciative when they win things, especially majors.
When I listen to commentators and fans sometimes, it leaves me with the impression that people don't fully understand or appreciate how difficult it is to do what Federer and Nadal are doing. Maybe it is because they make it look easy. I think the other players are all too aware though, and are looking to crash the party.
Players, at the end of the day, are not machines that perform with the reliability one can expect from a computer. They are flesh and blood, and can falter at any moment, as we saw with Federer on Sunday and Nadal in the first set. It is only when they are retired that they can look back on their careers without fear and doubt.
So to answer the question, the next non Federer/Nadal slam winner, can occur at any time. We won't know when, until it happens.