Various countries have different views on how long a controller can be in position for. In the UK for instance, units like Terminal Control at West Drayton and the Enroute sectors at Swanwick normally stipulate a controller will be in position for an hour and a half maximum then they must have a half hour break (tis the law
) Here in Canada there is no set time in position and can vary depending upon staffing levels or how may coffees you have had.
As to eye-strain, I didn't wear glasses before I started ATC.......but I do now
. Seriously though, I have found I only need mine for working the radar. The mix of those that do wear them and don't is very roughly about 50/50. And there are some controllers who work "procedural airspace" that use flight progress strips and don't have radar............but that's a completely different story.