Try this. To include the estimated time away, a user would type:

For 2 hours:
!afk -t2 reason for being away

For 30 minutes (0.5 hours):
!afk -t.5 reason for being away
!afk -t0.5 reason for being away

Etc. This is only in hours, so decimals are needed for minutes... or, you could enter it as:

For 30 minutes (30 minutes * 60 minutes in an hour):
!afk -t30*60 reason for being away

Code:
on *:start: {
  hmake AwayTable 50
}

on *:text:!afk*:#Simons-Forums: {
  var %time = $ctime
  if (!$hget(AwayTable)) { hmake AwayTable 50 }
  if ($left($2,2) == -t) {
    if ($len($2) == 2) {
      var %reason = $iif($4,$4-,Unknown)
      .timerAway. $+ $nick 1 $calc($3 * 3600) msg $chan $nick should be back soon.
    }
    else {
      var %reason = $iif($3,$3-,Unknown)
      .timerAway. $+ $nick 1 $calc($right($2,-2) * 3600) msg $chan $nick should be back soon.
    }
  }
  else var %reason = $iif($2,$2-,Unknown)
  hadd AwayTable $nick %reason %time 
  msg $chan $nick is away @ $+ $time(%time,hh:nn mmm dd yyyy)
}

on *:text:!back:#Simons-Forums: {
  if ($hget(AwayTable,$nick)) {
    msg $chan $nick is back from $gettok($v1,1- $calc($gettok($v1,0,32) - 1),32) $+ .  $nick was gone for $duration($calc($ctime - $gettok($v1,$gettok($v1,0,32),32))) $+ .
    hdel AwayTable $nick
    .timerAway. [ $+ [ $nick ] ] off
  }
  else msg $chan You're not away.
}

on *:text:!away:#Simons-Forums: {
  var %cnt = 1
  var %total = $hget(AwayTable,0).item
  while (%cnt <= %total) {
    var %name = $hget(AwayTable,%cnt).item
    var %data = $hget(AwayTable,%cnt).data
    write afk.tmp %name is away $chr(40) $+ $gettok(%data,1- $calc($gettok(%data,0,32) -1),32) $+ $chr(41) since $time on $date($gettok(%data,$gettok(%data,0,32),32)) $+ .
    inc %cnt
  }
  .play $chan afk.tmp 1500
  .remove afk.tmp
}

on *:text:*:#Simons-Forums: {
  if ($hget(AwayTable,$remove($strip($1),?)) && !$2) {
    var %data = $hget(AwayTable,$remove($strip($1),?))
    msg $chan $remove($strip($1),?) is away $chr(40) $+ $gettok(%data,1- $calc($gettok(%data,0,32) -1),32) $+ $chr(41) since $time($gettok(%data,$gettok(%data,0,32),32)) $+ .
  }
}

on *:action:*:#Simons-Forums: {
  if ($hget(AwayTable,$remove($strip($1),?)) && !$2) {
    var %data = $hget(AwayTable,$remove($strip($1),?))
    msg $chan $left($1,-1) is away $chr(40) $+ $gettok(%data,1- $calc($gettok(%data,0,32) -1),32) $+ $chr(41) since $time($gettok(%data,$gettok(%data,0,32),32)) on $date($gettok(%data,$gettok(%data,0,32),32)) $+ .
  }
}

on *:part:#Simons-Forums: {
  if ($hget(AwayTable,$nick)) {
    hdel AwayTable $nick
    .timerAway. [ $+ [ $nick ] ] off
  }
}

on *:quit: {
  if ($hget(AwayTable,$nick)) {
    hdel AwayTable $nick
    .timerAway. [ $+ [ $nick ] ] off
  }
}


As a note, you can enter it using a space after the -t if you want to:

For 3.5 hours:
!afk -t 3.5 Reason for being away.

Invision Support
#Invision on irc.irchighway.net