I've just installed GSX and I wonder if it's normal to wait such a long time (more than 30 minutes!) for the airport cache regenerating?
No, it's not.
When I've used the trial version I've instantly used GSX without delay at Munich airport but with the registered version I was unable to use GSX's functions
This is simply not possible. Nothing in the cache regeneration process could be possibly affected by the activation status.
and I'm just seein this awful message indicating me the airport cache is regenerating. I've reached so far 83%... I just want to know that is it because it's my first use?
It's not normal is taking so long, and it's normal you see the message depending if you have changed something in the Scenery Library or not. If you have changed something in the Scenery Library, it's normal the cache should be regenerated, but it's not normal is taking so long.
If, instead, you haven't changed anything in the Scenery Library, it's not normal you would see such message UNLESS you closed FSX before allowing at least one cache regeneration to complete because, in that case, the regeneration will restart from scratch at the next restart.
I've seen the mesages about Windows Defender and FSX.exe is not included in this software processes...
Excluding the fsx.exe doesn't have anything to do with this, if anything, you should exclude FSX\fsdreamteam\Couatl\Couatl.exe but, it's best if you exclude the entire FSX folder from scanning, otherwise the cache regeneration will be A LOT slower.