that depends a lot of what you are doing during the break. In a worst case scenario (bed rest), muscle atrophy starts almost immediately. Here is a quote from a study published by the NIH:
” In the absence of a robust countermeasure (nutritional, exercise, or pharmacologic), lean tissue loss is largely inevitable during prolonged bed rest. Inactivity-induced loss of muscle mass predominantly affects the lower body musculature and is most rapid during the initials days/weeks of inactivity [9-11]. In young, healthy adults subjected to bed rest, the loss of lower body lean mass appears to be on the order of 100-200 g┬Àwk-1. LeBlanc et al. reported a total body lean mass loss of 2.6 kg following 119 days of bed rest in healthy young males; of this, 2.4 kg was from the legs (0.14 kg┬Àwk-1) . In a similar cohort, Paddon-Jones et al. reported a loss of 0.4 kg of lean leg mass following 28 days of bed rest (0.10 kg┬Àwk-1) . More recently, Trappe et al. reported a 21% (quadriceps) and 29% (triceps surae) decrease in muscle volume after 60 days of bed rest in young females .”
Here is a link to the entire publication http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3276215/.