Your body will start braking down your muscles right away (start losing gains about 24 to 48 hours post exercise). There are alot of factors.
*your genetics, DNA
*your activities, exercise, work,
*your nutrition (foods), macro's, supplements.
Don't be narrow-minded and stop doing something important because you will be 1 week or 1 month away from the gym... Can be work, vacation, recovery, surgery, anything. Do what you have to do.
Your body is not dumb and if you stick to the diet, you will not disappear or blast if you stay away from the gym. There's life, it happens and sometimes we have to give up something but the minute you get back, you'll be back on track quickly.