I change mine about every 6 months regardless of how they look and sound. But I use Elixer NanoWebs, they sound great to me and I don't need to change the strings as often, its a job I don't like doing.
I use Elixir Nanowebs on all my acoustics because there coated they last for about 6 months or even longer. I have 6 acoustics and play them all, I don't want to change strings any more than I need to.
I love Elixir Nanowebs also when you have a number of guitars the extra life that these strings offer is a great addition to the main benefit, the way they sound. I average about 6 months between string changes.