When you have a small window of time, it's important to use it at the best of your ability. Some people have jobs that last half a day or more and exercise might be the last thing on their mind but it can be done if you make the effort to do so. I knew people that have made so many excuses but never once had a "I'll make the effort" comment. Sure it takes discipline and understanding of timing and being fresh as best as possible but overall it's got to have meaning to you. Most people these days work long hours and don't have time for the gym to get in a solid hour but there are possibilities.
I learned from a friend a few years back that when you have that short window and it can be used for exercise, take it. He is always on the move and is a performer in stage shows and produces/manages many people so what the hell does he do for exercise before coming home waiting to pass out? He exercises little by little throughout the day. When someone isn't hassling him or has a window away from the laptop in his office, he does Isometrics and or a bit of calisthenics (using his office chair, desk, sidewalk if he has to) and even when he's at a stoplight on an intersection. He makes the little time he has to train his body and it's kept him energized throughout the day.
Sometimes I have more time than most because I work for myself but there are days where I'am very busy doing articles, being with my girl, dinner gatherings, going out to places with people and traveling whether on the ground or in the air yet I make the most of what I can do throughout some parts of the day; i'll do push-ups or squats at a gas station while the tank is filling up, do some (what I call) Playful Movement or do Isometrics while someone is driving, rise up on my toes in the grocery store, do deep breathing exercises waiting in line for a movie, all sorts of stuff that is convenient and not looking too obvious. Make your timing count when it needs to be. Even do little things between commercials if you're watching TV (they suck anyway) or a small routine while you're in Bed (not doing the horizontal mambo). There's no reason why you shouldn't exercise unless you're in severe dire straits and about to die. You are your own gym and you have the ability to make things important to you, do so and make it work for you.