When should you decide to cancel a date or plan?

Last night, my girlfriend invited me to her place after her work (, gets off at 10)
It was 11pm before she said she's almost done, and so I canceled our plans. I felt like it was too late and I didn't want to wait around anymore.

She was pretty upset at me, but I felt like I was wasting my time waiting..

What would you have done?
When should you decide to cancel a date or plan?
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