Well say it takes you 5 seconds to switch, then you have to switch back so that's another 5 seconds. Multiply that by a couple hundred times, and you're looking at a half hour of time over the day just switching your glasses.
Yeah, it doesn't seem like much doing it a couple times, but if you have to do it a lot over the course of the day it really adds up.
Personally I was surprised myself how much it ended up when I ran 10 multiplied by 200 then divided by 60. It came out to 33.3(repeating). I wear glasses myself, but I never really thought about the time it would take to remove them and put something else in place.