Canoe trip this weekend - how to decide how long it will take

it depends, are you just straight up paddling? are you stopping much? are you fishing?

will be kayaking Amite on Saturday as well, but much further downstream

Assuming the river you're floating has a relatively slow current, since most in south LA do. I would say that for a full days float (6-8) hours, 7 or so miles should be about right. If there are road bridges or railroad bridges that cross your route, that should help you gauge how far you've gone. It is kind of guesswork, and I'm no expert on the topic, although I have done dozens of float trips in my canoe. Worst case scenario if your route is too long is that you have to paddle hard at the end to avoid nightfall...which I've certainly done. If you have somewhere to be, then I'd go shorter than that...but if none of yall have plans after the trip, then I'd rather it be a little longer than expected than not enough time on the water. That's my take anyways.