Basically there's an expected time,…

Basically there's an expected time, say ~10 minutes (though it's 9.something really). And at any time, the expected time is exactly the same.

Therefore it's 50/50 if the next block is in 10 minutes. But if you've waited 10 mintues. It's once again 50/50 that it will be in the next 10 minutes.

The expected waiting time is always 10 minutes.