I can't say I've ever noticed callouses from guitar playing. I did play the viola for a long time before and during my time as a guitarist, so that might have had something to do with it.
I have far, far worse callouses from rowing, and something tells me they'll never go away. It's been several years since I stopped already, and they've not gone down.
That said, I don't really give a flying funk. Who cares if I have a few callouses on my hands?! Oh my, I can't let them see me like this! Jesus.