You can try a ctcp finger, which will return their network idle time, however, they will be alerted when you do so.
/ctcp NICK Finger
If you want to avoid the alert, you can try using the 'idle' prop on the $nick identifier, but I'm pretty sure it only returns their idle on the channel specified. To use this, try: $nick(#chan,NICK).idle
For example:
$nick(#mirc,HAMM3R).idle
It will return their channel idle in seconds, so to format it using minute/hours, use $duration.
$duration($nick(#mirc,HAMM3R).idle)
Good luck.