So where do I add it in then?
In your console as
$Game::Item::MaxRespawnTime = timeInMilliseconds;. You could put it in your prefs but they'll just be rewritten anyway (and it won't include your $Game pref since it only exports $Pref::Server::* stuff)
Is there a more simple way to do this like an add-on that does it automatically for me so I don't have to do it myself manually like every day?
No add-on is made to do this so it won't be available until someone makes it. I looked into the DSOs it seems it just initializes it so if someone basically just puts "$Game::Item::RespawnTime = timeMSHere;" in their add-on code it won't be rewritten (meaning it will work just fine when you the server loads). All you need is 1 line of code (from above) and it should work fine, unless the code that does the respawn time stuff has some stupid hard limit.