After I have scanned the QR code and received the text for entry, do I have to show up right away or can I show up sometime within an hour or two?

You have 1 hour

You have a 2hr window to come

Like the others said, you have an hour. They were also able to give me an approximate time for our callback based on what number in line we were.

I believe it was an hour window after the text is received.

You have an hour window to get back to get in.