Being available is half the work when it comes to finding success as a model and being a model! If you can manage to make your schedule available as possible you will be able to get more modeling work. Agents really like it if you are available and able to go to a casting last minute. If you do happen to miss a casting or can't make it then don't think it is the end of the world, these days being a freelance model and working with modeling agencies is very popular. It isn't like all models are put up in houses and have castings every single hour of the day. Many models model part time, and find great opportunity, but having a flexible schedule is best. Usually castings can be at the start of the week and the shoot is at the end of the week. It depends. Sometimes I have gone on castings and then didn't find out I booked it for up to two weeks! The photo shoots for the modeling jobs usually happen duirng the week and can last all day. I have been on shoots on Saturday as well- I did this for a Time Magazine shoot- so being available anytime really helps!
Your agents will call you more often also if they know you are available. And it is a good idea to keep in touch with your agents, and to communicate with them as soon as possible if you know you will be out of town.