We have upgraded our CF licence from CF2016 (Standard Edition) to CF2018 (Standard Edition).
Is it allowd to use both servers in parallel until the migration is completed (and how many weeks do we have)?
This is not really a great question for a developer support community forum, unfortunately. Most of us aren't lawyers, and you'll have to take our answers with that in mind. In general, though, Adobe has been pretty good about allowing concurrent servers to run as long as only one of them is really "production". That's what it sounds like you have going on here. I mean, you're not bouncing live users from one server to the other, right? It sounds like your users are still hitting your CF 2016 server while you're testing against your CF 2018 server. So, you should be fine (says the non-lawyer).
Another thing to keep in mind is that CF provides a "trial" license that lasts for 30 days. During this time, you can use all the features of the product. This is actually a time-limited Enterprise license, so you'll be able to use more than you can in your existing CF 2016 production environment. But that shouldn't really be an issue - just plug in your new Standard license number when you're ready to transition.
Finally, there's nothing in the CF license software that actually prevents invalid use, as far as I'm aware. Maybe I shouldn't say that here, but that's been my experience.
Dave Watts, Eidolon LLC
And just to add clarification to Dave's excellent reply, if you DID go with the trial version, when you were ready to "go live", you could then just put the Standard license key into the CF Admin (via the "system info" button in its top right corner.
But something to beware is that the Trial edition (like the free Developer edition) runs as if it is Enterprise functionality, so when you DO drop in that CF Standard license, it will switch immediately back to Standard-level functionality. No restart needed. But just be careful during the trial not to take unexpected advantage of "Enterprise" features. That won't be the case if you are only doing testing, as you propose, Eric
Finally, if you wanted to see what Adobe says (about whether your use for staging or testing requires licensing), the licensing agreement is in the root of the CF installation directory. More here: https://coldfusion.adobe.com/2018/01/finding-the-eula-end-user-licensing-agreement-for-your-installe...
Thank you Dave and Charlie for the prompt and precious 'non-lawyers' replies.