Stritix

Why is there no setting to change this? Basic functions from IPB3 are missing in IPS4. I just upgraded to IPS4. I'd rather not modify files. Please let us change the cut off time.

I can nearly guarantee a plugin that simply selects from the user table to determine who has been active in the past X hours to manipulate the list is going to be more reliable than hacking code, changing server configs, etc.  I'm not sure you understand what you're actually doing here - you're not setting a display cutoff, you're altering the PHP garbage collection config. 
We've had issues with people tinkering with this and I feel compelled to note, you're "voiding your warranty" so-to-speak and we will not provide tech support should you end up with "people aren't staying logged in" etc. Proceed at your own risk. 

Because IPS4 uses PHP sessions and the default garbage collection is 30 minutes. Editing PHP configurations is beyond most users and and it's really not necessary - if you want to make your site look more busy than it actually is, there's plugins in the marketplace to replace the online user block. You should ideally leave the stock settings alone.