I had my JOD iPhone 6s+ fully unlocked after 40 days with T-Mobile. Same with my wife's 6s. But we were new customers then.
All you do is insert the new SIM, you're asked for your Apple ID details and that's it.
A 'temporary' unlock sounds like they are only giving you an international unlock. I'm not too sure about 'temporary' as that doesn't really apply to iPhones. It's more an Android/Windows phone type of thing, but maybe they have some way of only making international temporary now - IDK.
If you don't have the SIM you are going to us abroad and you want to make sure you are unlocked, then restore your phone in iTunes (when they tell you the unlock has gone through).