According to Osho, ‘science’ has levels: physical science worries about ocular strain, but spiritual science knows the third eye. If the practice is done with deep inner resolve to awaken that energy, even four hours’ open-eyed meditation causes no harm and may strengthen vision as a by-product. Without such resolve, strain can harm—but then the eyes usually close before forty minutes.
With strong spiritual intention, open-eyed meditation won’t hurt your eyes and may help; without it, you’ll strain them or they’ll just close.