I haven't seen or found a setting the enables or disables the Last Active time on the Facebook apps. From my own account, I can only see the last active time for some friends, which I believe Facebook has determined that I'm close with. I know for a fact other friends have iPhones with Facebook app installed and I don't see a time next to their name, even though I did in the past. I think Facebook is continuing to tweak the Last Active time setting for users, so I'm guessing this could change.
With that said, I do know that if someone doesn't use the Facebook app / messenger app for two days, the Last Active time disappears (It use to disappear after 4 days).