Just FYI, RM2 mandated that characters spend 1 exhaustion point every 6 rounds of Concentration -- that was the same as missile fire. So the intent seems to have been that concentration was about as fatiguing as shooting a bow. An average character (50 exhaustion points) could go for 300 rounds before being exhausted). I know you're not using exhaustion points, but did just want to lay out what the original parameters were, for a baseline on balance issues.
As for alternative solutions, I really like the RMU system: it just calls for checks if you are doing something that could reasonably exhaust you. They have used the same basic equation for Concentration, but now instead of spending a point after 6 rounds, you just make a check. The new system means you don't have to keep track of anything, and there are gradations (-5, -10, etc.) along the path to absolutely exhausted.