Hello,
I am one of those people who can get by doing each body part once a week, except legs which I do twice. I make sure I lift as heavy as I possibly can and make sure I reach muscular failure each time. If I do body parts twice a week, I feel like I can't lift as heavy and I don't feel fully recovered. That is just me though, I am an endurance runner so I also save a lot of my strength for that. I feel very strong and my arms have nice shape. As long as you don't want to gain muscle, lifting once a week I think is sufficient and you won't lose strength. Sometimes however, I will do Circuit Max, but that is just lighter weights, higher reps. I never lift heavy twice a week for my arms. My best friend is a personal trainer and she only works her arms once a week too. Although, everyone responds differently, this weeks for me. Just my humble opinion.
Take care!
Christine