There is evidence that increasing the Length of Stay (LOS) contributes to the sustainability of destinations. This paper analyzes LOS determinants in the Schist Villages Network (SVN) in Portugal, where almost half of the visitors are same-day visitors. Given that these visitors generate more environmental impacts, an increase in LOS would help guide tourism towards more sustainable standards. The analysis was performed using several Heckman selection models. Unlike the findings of previous studies, LOS depended mainly on the characteristics of the trip and the destination—not on visitor profile—and can therefore be considered a highly specific form of tourism. Comparatively, motivations associated with rest, residents’ hospitality, and local produce consumption encourage visitors to extend their stay, thereby boosting tourism sustainability. The SVN is also shown to have various cultural and heritage resources that are currently highly valued. However, the leisure offer is insufficient to increase LOS. Furthermore, conventional advertising was not shown to be effective in raising LOS compared with word-of-mouth (WOM). The paper concludes that measures should be adopted to increase the local produce offer, retain population, maintain traditional village activities, and attract visitors from nearby regions. These actions could increase overnight stays and LOS, contributing to more sustainable development.