I think he peaked at the AO 2012. Shame it wasn't enough to beat Djokovic and wouldn't have been enough to beat Nadal in the final anyway.
In recent months he has looked out of form physically even if he is mentally starting to improve.
I think even when he beat Djokovic, that was a case of Djokovic having a bad day at the office rather than any blistering display of form by Murray. Murray looked poor throughout that tournament, and his decline was highlighted by Federer's beatdown of him.
Historically, grinders like Murray have always peaked early (see Hewitt).
Can we expect Murray to steadily fall down the rankings in the coming months and years?
Yes. This time next year Murray will be nothing, occupying the lower echelons of tennis, when you begin searching for another player to waste your life trolling over.