Here's the thing: A Soother or Rioter isn't going to be burning constantly for hours on end, but rather giving little nudges here and there. I'd say, for those two metals at least, use the given charge time or one charge per beat, which ever is longer. Not sure if that'd break anything...
Not likely, but I'm unsure how burning works. I don't think its within the rules to stop burning a metal midcharge. In my campaign they can, but not once its been flared. I however, DM by the seat of my pants and ignore rules to a flagrant degree. Instead I follow my gut.