You can expect the balance to be shifted heavily to work. They will call your cell phone and ask if you can work on your day off. They will ask you during your shift if you can spend an extra 3 hours that day. And you will be required to work Black Friday. Any availability you give, they will schedule you for, and sometimes they will schedule you beyond that.
As a starting sales associate you will most likely get only four hour shifts. It all depends whether the Macy's your working at has a part-time or full-time shift open. In my opinion, time passed by pretty quickly for me but that was because I tried to keep myself busy in any way. I did take a look at my fellow co-workers schedules who have worked longer than me and it is quite overwhelming. I will say that Macy's is great for college students looking for part-time, but full-time would be asking for too much.
You give them your availability before you start and they develop a schedule based on that availability, but you should be prepared to work odd hours, including nights and weekends if necessary. Most people work part-time though and there is a lot of flexibility for students.