|MadSci Network: Botany|
Plants do not need sunlight if they have sufficient electric light. If you mean, how long plants can survive without any light then the answer varies widely depending on the species, size and other environmental conditions, especially temperature. Nurserymen used to overwinter evergreen plants in caves or barns for six months or so. In that case the temperatures would have been close to freezing. Of course, deciduous trees and shrubs could probably do without light for about six months per year when they have no leaves. Home gardeners sometimes uproot geranium plants (Pelargonium species) before a killing frost and hang them upside down in a cool basement overwinter, which is usually dark. They survive and can be planted again the next season after the danger of frost has past. At room temperature, I would guess a small seedling with its seed reserves exhausted might die after a few days to a week in the dark. Larger plants would take longer depending on how much stored nutrients they have. Plants with storage organs, such as bulbs corms and tubers would last longer than those without. Some parasitic plants lack chlorophyll so can go through their entire life cycle in the dark, particularly the underground orchid. However, they need a host plant that requires light to photosynthesize. References Saving Geraniums Over Winter Re: Are there plants evolved now that feed on other plants? Underground orchid
Try the links in the MadSci Library for more information on Botany.