That's a great question, Mike!
Coating mainly does four things: 1. Increases perceived color densities. 2. Helps eliminate micro-cracking when stretching. 3. Increases the durability (mainly scuffing and water resistance) of the print. 4. Helps inhibit UV light (it's of course best to opt for OBA-free media).
How do you tell if it's enough? The main way to do this is to just look at it, like you've been doing. It is safe to say that if you don't see any cracking when you stretch the print, then you have coated it well.
I often see multiple coats applied to try and maximize the color "pop" achieved when applying a satin or matte varnish. Be careful here though, as too much coating will start to knock down your highlights. I've generally heard that this will start to happen after about three light-medium coats (this can certainly vary depending on the specific media and each different manufacture).
Have you experimented with it much? How do your experiences compare, if so?