Reservation agent work normally 8 hrs full time and 4 hrs part time, rarely you will have an over time. Normally, the overtime is due to climate, too many flight cancellations, holiday season, etc. This mean you should be able to plan ahead of time your work-life activities. Even though work- schedule vary every quarter due to airport operations, but it should'nt be a big issue.
Personally, I was able to go to the gym and have night life while I worked a 8hr shift in the call center.