San Millan de Cogolla is about 10km off the Camino Frances, some three to four hours walk along countryroad from Azofra (Between Najera and Santo Domingo de la Calzada). It is at the foothills of the Sierra Demanda. I visited as a detour, starting at 05:00 in Azofra, arriving at 08:30. I joined the first guided tour in the mornin - big reduction for peregrinos with credentials - they also give a sello! Staid there twelve and arrived in Santo Domingo at 18:00.
There might also be Hostels or Albergues around (not in 2013). There are two monsateries Yuso and Suso. It is a World Heritage Site, and apparently quite busy during the day. http://www.monasteriodesanmillan.com/