How long to wait for a text message before making other plans?

So I asked my friend if she wanted to grab food later on today. But she hasn't responded yet and it's been an hour. I'm not one to expect replys back right away. However I have to go out to an appointment later and as I do not drive and live about an hour and half bus ride away from the city she is in I... show more So I asked my friend if she wanted to grab food later on today. But she hasn't responded yet and it's been an hour. I'm not one to expect replys back right away. However I have to go out to an appointment later and as I do not drive and live about an hour and half bus ride away from the city she is in I don't want to come home after my appointment just for her to text me and say she can go. This is also because the bus service is bad from my house.
So my question is how long do I give her to respond before it is acceptable for me to cancel on her if she does reply? Also what can I say to her that doesn't sound like I'm being rude?
Thanks in advance :)
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