There are various standards as to allowable exposure to avoid(or more frequently just minimize)noise induced hearing loss. It should be kept in mind that some of the more frequently referenced tables apply to occupational noise hazards which may actually allow for some hearing loss, but is considered acceptable(at least by some)in a workplace setting. The EPA and WHO set stricter limits for "community noise" (non-occupational)where any appreciable danger of hearing loss is viewed as unacceptable. This table
summarizes two occupational standards as well as the community standard which would apply, among other situations, to audio. Another source(Cohen, et al., Sound&Vibration, 1970)extends the range to 16 hours at 70dBA, indicating that at 70dB or below there's essentially no risk of hearing damage, regardless of length of exposure.