Thanks for the help!
I now have this, and it works:
Code:
on *:START: {
  set %pdeactivity 0
  set %pdewait 1
  set %rawblocked 0
  .timerremovewait 1 10800 { %pdewait = 0 }
}

on *:INPUT:#PDE: {
  if (/* !iswm $1) {
    inc %pdeactivity
    .timer 1 10800 { dec %pdeactivity }
  }
}

on *:TEXT:*:#PDE: {
  if ($nick != ChanServ) {
    inc %pdeactivity
    .timer 1 10800 { dec %pdeactivity }
  }
  if ($1- == !activity && !%rawblocked) {
    if (!%pdewait) {
      %rawblocked = 1
      if (%pdeactivity < 1) {
        notice $nick 14[12Automated message14] In the last 3 hours I recorded %pdeactivity messages being sent to this channel. (No activity)
      }
      elseif (%pdeactivity < 30) {
        notice $nick 14[12Automated message14] In the last 3 hours I recorded %pdeactivity messages being sent to this channel. (Very very low activity)
      }
      elseif (%pdeactivity < 75) {
        notice $nick 14[12Automated message14] In the last 3 hours I recorded %pdeactivity messages being sent to this channel. (Very low activity)
      }
      elseif (%pdeactivity < 100) {
        notice $nick 14[12Automated message14] In the last 3 hours I recorded %pdeactivity messages being sent to this channel. (Low activity)
      }
      elseif (%pdeactivity < 200) {
        notice $nick 14[12Automated message14] In the last 3 hours I recorded %pdeactivity messages being sent to this channel. (Average activity)
      }
      elseif (%pdeactivity < 350) {
        notice $nick 14[12Automated message14] In the last 3 hours I recorded %pdeactivity messages being sent to this channel. (High activity)
      }
      elseif (%pdeactivity < 500) {
        notice $nick 14[12Automated message14] In the last 3 hours I recorded %pdeactivity messages being sent to this channel. (Very high activity)
      }
      elseif (%pdeactivity < 700) {
        notice $nick 14[12Automated message14] In the last 3 hours I recorded %pdeactivity messages being sent to this channel. (Very very high activity)
      }
      else {
        notice $nick 14[12Automated message14] In the last 3 hours I recorded %pdeactivity messages being sent to this channel. (WALLOFTEXT)
      }
      .timer 1 2 { %rawblocked = 0 }
    }
    else {
      notice $nick 14[12Automated message14] Sorry, but I have been in this channel for less than 3 hours, so I cannot hand out precise activity information, yet. I have logged %pdeactivity messages being sent to this channel so far, though.
    }
  }
}