It's complicated, and probably varies from airline to airline. Usually you will bid on your schedules based on your seniority. As a new agent you'll get mid week days off. You'll usually be allowed make shift trades too. Basically airline's have very flexible schedules. You can fit the hours around your own life. I know people who live off of shift trades. Some people get together and work it out so that they're each working 2 16 hour days and 1 8 hour day a week. That's a 3 day work week but still 40 hours. My company is also very generous with vacation days. We start with the standard two weeks a year, then after 5 years it goes to 3 weeks, 10 years 4 weeks, 15 years 5 weeks. Plus on top of that you get one "freeday" per month. This is basically a vacation day that you can use for whatever every month. I get plenty of time to travel and take time off. I work for Southwest and we get treated VERY well though, so I can't speak on what other airlines offer their ramp agents.
It's a pretty good job if you can cut it. It's not for everyone, and you have to be able to tolerate some pretty wicked conditions and long hours. But in the long run, the pay is good (at Southwest we make $26 an hour) and the benefits are top notch. I've been able to travel all over the world. I can't imagine doing anything else for a living.