This setting can not be changed in the user interface but can be in Zoiper's config file. It's obvious that the developers didn't believe this to be a important setting or didn't want users changing it; but it can be changed.
The config file in Windows is here:
<USER> is the name of your login name or profile that you created.
Then, look for the file "Config.xml" open it in a text editor of your choice but I prefer Notepad++.
When opened look for this section: <auto_away></auto_away>
Then you can set it at a high number. I believe this number is in seconds. I set this a long time ago when Zoiper had a bug that when I had "Do not Disturb" set then when auto away would trigger putting the client in away status it would allow phone calls to come in again. I don't think this bug exist any longer so having the client go to auto away is immaterial except when your proxy responds to PUBLISH events.
You can set it to this: <auto_away>604800</auto_away> this well give you 7 days before the auto away triggers. Considering that you use your PC in that time frame; this effectivly disables it however it's not truly disabled.