There's nothing worse than trying to make gel with moss that was not fully hydrated. It's gritty and thick, and you don't get a proper yield. This sometimes happens when you're in a hurry.
An easy way to tell if your sea moss is ready to blend is with a quick pinch test. You don't have to choose the thickest stem, but you shouldn't choose the thinnest ones.
If it snaps when you pinch it, your moss is ready to blend. If not, add more water to cover and test it again in an hour.