If you want to make an exercise program a regular part of your life, it's best to assign it a particular time in the day and become accustomed to exercising then. If you're always used, for example, to doing your exercises right before hopping in the shower, it will increase the sense that something's wrong if you should skip them one day.
If, instead, you try to shoehorn exercising into a random snippet of ``free time'' every day, you're demoting it to a lower priority. Before long, it will become a duty to be disposed of at the end of the day and then, in all likelihood, discarded as one of the things you ``can't make time for.''
Certainly anybody with a harried schedule will have to improvise from time to time, and that's OK; it's far better to work an exercise session in at a weird time than skip it. But from day to day, try to find a time that fits your schedule and stick to it.