I have been using Elixir PB 10s on my 712 - but will soon be changing to 11s. Have tried D'Addario EJ16s but they were a little too heavy for the bass end and lost their tone quite quickly.
On my Martin I use Elixir PB nanoweb 12s - these suit the guitar beautifully, with a rich bass and clear top. I prefer 11s to play but had problems with D and G strings breaking when playing aggressively! The 12s seem to stand it.
I have tried most brands of strings over the last 40 years, including hand wound, and I can honestly say that, for me, nothing comes close to Elixir, they sound like I want strings to sound.
As for changing them, this depends how frequently I play and what style I am playing. The strings on the 712 last a great deal longer finger picked than the strummed strings on the 000.