How long to steam fish for?
When I steam fish, I usually steam it whole, but here are the steps I used to check for done ness

Are the eyes of the fish white?  A cooked fish's eyes will harden into white balls as the proteins in the eye coagulate

Can you separate the fish meat from the bone easily? Cooked fish should just barely stick to the bone and should come off very easily

Is the flesh easily separated? Using a chopstick or some other implement, the fish should "flake"; you should be able to easily separate the "sections" you see in the fish without much force

Is the flesh a solid colour or translucent? Cooked fish should be a solid white/pink/whatever colour and not slightly translucent like it is when uncooked

Of course, only the last 2 steps will probably apply to your fillet, but hope this helps!