It varies between the times it takes the screen to go black but I cant find anyway of turning it back on. Only thing that works is turning the power off but this cant be good for it. Sometimes in wont turn back on or lasts only a few seconds before switching itself off again. On start up it says the battery is running at less than 25% so have to use it plugged in. Doesn't happen when using all programs jus some. Please help. Doing our heads in!!