Yes, as little as 6 hours of bed rest begins the process, however the the real significance is the effect of atrophy over the long term with possible concomitant problems such as: cachexia, sacropenia.
Additionally please read:
Pavy-Le Traon, A. (2007) From space to Earth: advances in human physiology from 20 years of bed rest studies (1986–2006). European Journal of Applied Physiology
SpringerLink - Journal Article
Alkner, B. A. (2004) Efficacy of a gravity-independent resistance exercise device as a countermeasure to muscle atrophy during 29-day bed rest. Acta Physiologica Scandinavica 181(3)
Prolonged bed rest decreases skeletal muscle and whole body protein synthesis -- Ferrando et al. 270 (4): E627 -- AJP - Endocrinology and Metabolism
Calf muscle area and strength changes after five weeks of horizontal bed rest -- LeBlanc et al. 16 (6): 624 -- American Journal of Sports Medicine
Also, please consider time post-incident. Most patients I saw while I was on Orthopaedic ward would have 2-7 days before operation, and depending on co-morbidities another 2-7 to begin mobilising.
However for your patient, previous activity/strength levels are equally important.
I would consider rapidly progressive isometric, to functional isometric, to weight bearing based on consultant report asap.
Additionally to back up the 6 hours:
Related Literature Skeletal muscle atrophy begins within 6 hours of bed rest (Booth, 1977; Booth & Seider, 1979).