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Longer than most people think!
TV screens 85 years (Europe, 80 in USA), though the CRT is a Victorian era invention over 120 years ago, simultaneously in England and Germany.
First computer screens probably about 60 years ago.
LCD screens about 40+ years (mono reflective for a long while). Plasma screens about 45 years, though only amber. Colour was much later.
Mobile phones for about 40 years, though first demonstrated in 1973.
Smart Phones (mono till about 2002) over 20 years (1998)
Apple iPhone was very late to the party and the ONLY innovation was the cheap data package. All bought in tech. Till then only rich people or companies could afford data. The iPhone originally was slower than competition as it had no 3G. Also the capacitive touch had existed since 1980s, but it's dramatically poorer resolution than resistive. Resistive was used because it allowed annotation and handwriting. Mobile phones even had fax before iPhone, which with resistive input allowed written Asian language messages.
So we'd know if people were going blind. Even the X-ray scare of CRTs proved false. Glass too thick.
Staring into a Radar waveguide will give you cataracts, not cancer, as it's about 800W average and 10k W peak power. The microwave oven is essentially using the same kind of device as WWII microwave radar, a Magnetron diode valve.
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