Sometimes in the am - just a few plank sets, hundreds and ab things - 5-10min tops
Sometimes during nap
Sometimes in the evening (really only walking, pilates or yoga to settle down)
As long as I do something, sometime each day, I don't beat myself up about it.
If you workout at night, try ending with some stretching, deep breathing and yoga poses that slow things down (child's pose w/forehead on mat, corpse pose) to bring your heart rate down so you can sleep better.
ETA: Ted's mommy is exactly right! Don't forget the cumulative effect of the activities you naturally do all day long - it adds up!