How much screen time should be allowed per day on mobile phone for 2-5 yr olds?

Call me old-fashioned, but I don't think children should be given mobiles phones at all. At least, not till they turn 10. I know it's really tough given the world we live in, but as responsible parents, we must practise some restraint. Besides, there's so much I keep reading about how mobile phones and e-toys can affect children emotionally and psychologically. Here's one such article: http://www.theindusparent.com/e-toys-hamper-kids-language-development