It depends on the studio/gym. They should clean them. I have my own mat, so I bypass that concern, but one studio I attend has a strict cleaning schedule for the mats, props, and floors. As anal as they are about it though, it still wouldn't be enough for me.
Honestly, if you think you might take a yoga class now and then, I suggest buying your own $20 sticky mat (can be found at Target, Walmart, Amazon, etc). If you plan on taking yoga more often, then invest in a good (and a bit more expensive) mat. It's worth it.