There is a saying; "If you don't use it, you lose it'. Once a week is not going to have any noticeable effect, however like spived2 said, any exercise is better then no exercise. Cardiovascularly it will benefit you to do anything that gets your ticker pumpin harder
. Keep your one day a week and try to squeeze in 30 minutes on a few other days when you have the time. Oh, and the days that you actually can workout, make sure you kick your ass at the gym...punish yourself hard to get better results.