In my area Frontier will chance an IP usually once every several weeks. It's started to become a bit more often (basically one chance once a week) but I assume this is due to them performing work in my area during the middle of the night, killing the PPPoA server for a few minutes. If you check your modem logs, you can find out when a new PPPoE connection is established, and when the DSL goes down/up and base your chances of the IP changing on that. You can log into your modem at »
192.168.254.254 and the username and password is admin/admin if you are using the SpeedStreams.
A DynDNS account is pretty easy to set up and can be set up within 10 minutes. The SpeedStream modems that Frontier hands out are capable of acting as the updater client for DynDNS (I use it on my SpeedStream. Checks for update every hour as indicated by the log, and updates the DNS when the modem is booted or the IP changes), so you won't need to leave your Mac on with updating software in order for updates to happen. For your own domain, I'm pretty sure DynDNS can set you up with something. They do hand out free ones which are something such as yourdomain.homeip.net but I'm positive you can get your own unique domain name none the less.