Daily, because I live with my family.
If I exclude them, then very seldom, unless you count the second it takes me to say hi (and only hi, and only if they say it first) when I occasionally pass a neighbor.
Last time would have been a year ago, but that had the intention of solving something. A conversation I had 2012 had the purpose of getting some information.
Going by SplendidSnail's good definition "Conversation for the sake of being social that lasts more than 30 seconds", then my last time must have been in 2003.
I can't say that isolation makes it harder for me to talk to people, I have never had anything to say when I have nothing to say, and I never can come up with anything to say or ask or comment. When the well is dry, it's dry. Been that way since childhood. It doesn't bother me much because I can count on a hand the times I have wanted to be social with someone. It has probably bothered those who have tried to engage me in conversation but that is their problem.
So for me it only bothers me when I actually want to say something, we're talking ca once a decade here. Then it does bother me somewhat but not to the extent that trying to practice would be worth it - it just be a very boring and uninteresting chore. The less I have to deal with people, the happier I am.
Quiet Is the New Loud 