STEP 3: Determine the length of your workouts
Estimate the minimum amount of time you'd be able to exercise on the busiest of your training days. That is, base it on the worst-case scenario. Using the example in Step 2, let's say Saturday is pretty open, which narrows down the toughest days to Monday and Tuesday. Maybe you like to exercise in the morning before work, which isn't a problem on Tuesday but is almost always an issue on Mondays because you have trouble getting up on time. You've just identified a potential problem.
The solution, then, may be to exercise during your lunch hour on Monday, or after work. Still, both could be time crunches, since you'll need to factor in a shower if you exercise during lunch, and there's a good chance something could come up unexpectedly if you wait until after work. So base it on what you know: the amount of time you'll have to exercise during your lunch break.
Start with the total amount of time you have and then subtract any time that doesn't contribute to your workout -- for instance, getting to the gym, changing clothes, and showering. The number that's left over is your "base" exercise time that you'll try to achieve in each workout throughout the week. If you have time to spare, that's great -- you can always exercise longer than planned. But this approach ensures that you have a concrete minimum.
Keep in mind that your life might be a lot different than the example. So the key is to think about all the possibilities and variables you might face ahead of time in order to give yourself the best chance for a winning game plan.