While I didn't find an identifier for this, it is returned as $3 in raw 317, which, in turn, is generated by the /whois command.

Usage: /server.idle <nick>
Note: <nick> is optional and will default to your nick if not supplied.

Code:
alias server.idle {
  .enable #server.idle
  .whois $iif($1,$1,$me)
}
#server.idle off
raw 317:*:{
  echo -a $duration($3)
  halt
}
raw 318:*:{
  .disable #server.idle
  haltdef
}
#server.idle end