If you do it correctly, within 5 seconds of the "bonk" (a hard hit between the fish's eyes), you have the fish in a 90%-100% painless state (brain dead) BUT the heart is still pumping the cardiovascular system like crazy for another few minutes (PLUS hold it by the tail head down to allow gravity to help).
Salmon are relatively small animals by mass and do not require their cardio system to be working for hours (a newbie assumption) in order to be bled.
I always carry a fish bat with me so that 1 to 3 (depending on fish size) well placed bonks kill the fish quickly. You can tell the fish is dead from looking at its eyes (they appear to be "looking down" when its alive, but "level out" when it's dead). The fish can be brain dead but as DennisK says the heart can still be pumping. Pulling the gill shortly after death often results in blood coming out in spurts, especially with the larger springs (sorry if I grossed anybody out here). Note also that a salmon, particularly coho, will continue to flap around for a very long time even after it's been bled, so don't keep bonking it until it stops moving. I've had coho start flapping around on the bank more than an hour after catching.
Tip: if fishing on a steep bank, don't leave your dead coho on the slope unless its tied to something, or this flapping can cause it to fall back into the river, and bye-bye coho (I've seen this happen).
Many times I've seen fishers trying to bonk a salmon with a fist sized rock (over and over). This just doesn't work very well (my bat has been loaned many times). Also, I've seen fishers bonking their salmon (again) long after catching them because the fish has started to flap around and they think it's still alive (wrong - it's the fish's nerves). I've had coho lay still for over an hour, then suddenly the tail starts flapping.
So please take a bat or bonker with you, and remember it's the fish's eyes that tell you it's dead, not the cessation of movement.