We used to buy 2-3 per year, then it increased to every 3 then every 2 months. I's clear that some time of the year we use the gas more often than other times but on average I am fairly certain that they run out far more quickly than they ever used to.
A friend of mine who lives in Fethiye but moves to Ovacık for the summer claimed her bottles there lasted less time than the ones she bought in Fethiye.