there are a lot of factors that would change the outcome of this answer. how good is your diet? that will determine how much you can gain on cycle and how well you'll be able to maintain it.  how long have you been training? someone who doesnt have a lot of experience in the gym often lacks consistency. take a couple months off from training because work got busy or you felt dedpressed over a break up and say goodbye to your gain. how much muscle do you already have and how close are you to your genetic potential? this would be rare, but if you were already at a point that you couldnt put on and hold your muscle because you're close to reaching your natural limit, anything beyond that point is going to be impossible to maintain after you come off the gear. can it be done? sure. especially someone who is new and untrained, as long as they keep training and eating right after the cycle. 

there is a huge mental factor involved with the idea of "just one cycle". steroids may not be addictive but the gains are. i've met a lot of people that say they're only going to do one cycle but i have never met one person who stopped there. often, those same guys are already planning cycle 2 before cycle one is complete! if you take bodybilding seriously, it can be very hard to turn back after seeing a years worth of gains in 3 months. steroids are a lifestyle. 

also, i should note that no matter what you do, the quality of the muscle is never going to look as full, hard and vascular off of gear.