How long does it take the Genius Bar to replace the glass screen on a iPhone 5?

For Friday, Sep 29, 2017 at Apple Aspen Grove in Littleton, Colorado - not sure how long it took to actually change the glass (no water damage, etc., glass only), but I waited just over 2 hours. If you're able, make the call directly to get a better estimate of the wait than what the online scheduler will give you.

The original appointment was made for 3pm-ish, during one of their slower times of the day.

Call, and ask.

Depends on how busy they are. I had it replaced in less that 15 mins.