Keep the %OnAir.* vars set to just a ctime value and use $duration() in the Info block.
Code:
;Starts the counter
; If you're only using a single 'word' trigger (!ppp, !ooo, !iii) you don't need the wildcard match ;)
on *:TEXT:!ppp:#:{
  msg $chan $nick is now on air!
  set %startctime $ctime
}
;Stop the counter
on *:TEXT:!ooo:#:{
  inc %OnAir. $+ $nick $calc($ctime - %startctime)
  msg $chan Time: $duration($calc($ctime - %startctime))
  unset %startctime
  msg $chan $nick is now off air
}
;Information
on *:TEXT:!iii:#:{
  msg $chan Time: $duration($($+(%,OnAir.,$nick),2))
}


You will have to reset the %OnAir.* vars that you have already.