How long is too long for texting back?

How long is reasonable to expect a text back from someone you're dating before you assume they're not interested, using you as a backup, or whatever?

When it comes to texting I don't care if someone doesn't respond right away. I'm busy and I assume other people are busy too. However, at somepoint it gets obvious your not busy anymore and that you just dont care.

I check my phone at least once in the morning, lunch, after work, and at night. I pretty much always respond with ~4 hours. Not including sleeping hours. If I someone texts me late at night then lunch the next day is usually latest I respond back.

The reason I ask is because I had a 4th date last weekend with a girl. She seems to really like me and wanted to schedule our date at the end of that date and she wanted me to know when she's leaving and getting back during Christmas. We planned on Thursday at 6, and I told her I'd plan something fun and I'd text her where to meet up. I texted her the next night Monday and she didn't respond until tonight.

Is that a red flag and should I not go out with her? Two days to respond seems like a good way to tell someone you're not interested.
How long is too long for texting back?
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