Where, or how, do I set it up in MSN Messenger or Yahoo! Messenger to automatically switch my status to either "Invisible" or "Offline" when idle for a half hour, or an hour?

I know how to set my status as "Away" or "Busy" after 10 minutes, but can't seem to find a way to set the offline status options without manual intervention.

Back story

As a software developer, I am very used to turning the computer on for the whole day and not turning it back off. (For example, checking email for urgent fixes, fix issue and push to web server). It's not even turned off when heading to sleep in case I might find it hard to fall asleep and come back to check on the computer. Or to have it there ready in the morning to check that everything is okay.

If I'm seen as being online for 24 hours of a day, some people see me as weird. Their perception of my value decreases as I'm always there (hard to get = high value; always there = low value).

Leaving it on makes everyone in my contacts list think I have nothing better to do all day than sit in front of the computer. Even though it's my job and I do admittedly spend more time online than other people. That's why I'd like to find a way to set my status as Invisible or Offline.