Goals have to be realistic and achievable, including the time you decide to commit to working out. One of the most important aspects to achieving your goals is consistency. In order to be consistent in your workouts, you have to choose a number of days and number of minutes a day that is manageable for you; it needs to be something you can stick with. Think about how your week normally goes, and which days are realistic for you to fit in workouts. Then decide how many minutes in those days you not only have time to workout, but are willing to workout. Whatever you decide on, hold yourself accountable to it. Keep a chart and track the days/weeks where you were successful in sticking with your plan. If anything, start small, and progress as you become used to making the time to workout. For example, you can find 10 minutes out of your day to do some sort of workout...so start with that, and progress from there.
It also helps to remind yourself why you are taking the time out of your day to workout. What is the goal you are working on? What will it do for you to achieve that goal? How will it make you feel when you get yourself there? How will it improve your life once you get there?
Make sure you do workouts that you actually enjoy. It's hard to be motivated to do something you hate. Some people get a rush out of running, while others would rather do just about anything besides running. Perhaps you like using the TRX, or enjoy exercising outside. Whatever it is, add something into your routine that will allow you to look forward to it, and stick with it. If you haven't found what you love, keep trying new activities until you do!
Here's a seven minute workout you can do if you are pressed for time.