Welcome to the Flow forum!
As far as harvesting goes, you should treat Flow frames like your traditional frames. When they are 90% capped or more, you can harvest and be pretty sure that you have ripe honey.
I like to pull my Flow frames out of the hive to inspect before harvesting, because my bees tend to do sneaky things like leaving an arc of empty cells at the bottom of the middle frames. 
You can’t see that from the end window. There is much more risk of a leak into the hive if those cells are not capped. If you have a hybrid super, you may find it easiest to lift out just the middle Flow frame for inspection, then just slide the others sideways so that you can look down at the other frame faces from the top of the hive.
When you come to harvest, I strongly recommend that you open each frame in about 20% sections, waiting for 5 minutes or so before opening the next section. If you don’t do that, there is a risk of an airlock forming in the Flow tube, which can quickly flood your hive with honey. Give yourself plenty of time too, perhaps start late morning and be prepared to let the frames drain for 2 to 4 hours. The last honey that comes out is the thickest, and that is the good stuff, so it is worth being patient! 
You will need a container with up to 3 quarts capacity per frame, unless you are prepared to switch the containers. I used 64oz mason jars, and each frame filled 1 and 1/2 jars.