One is experienced when they figure out how to create and develop a character and they stop asking (continually) for stats to come back and a respec.

But with more stats, one can gain more experience.

I have three different characters and they are all in the forties where levels are concerned but it seems that almost everytime I pick up skyrim and play I learn something new. I don't think you can ever be an "experienced" skyrim player

You eventually learn everything aside from a few unimportant features

That first time you kill a random npc...and he gets back up and kills you instead.

You either know the content or you don't. My first two characters were Mages which I managed to get to level 50+. On my third character I found a Hawk Beak alchemy reagent which I never saw before.

Even now when I have several hundred hours in Skyrim I still discover sometimes new things and I've had two characters with all quest lines complete and all dungeons cleared that are possible to.

Sadly around here you're going to get responses like the first page. People are stuck with the mentality that Skyrim is some game with infinite content that can never be completed just because it has no "YOUR TEH BEST" pop up where you can no longer continue to play that character.

The game may never tell you that you've completed it but you sure as hell can experience everything it has to offer in 150 hours.

My third character is approaching the point of having nothing but generated radiant quests left in the game and I'm nowhere near 1000 hours. I am very experienced when it comes to Skyrim.

Therer are many arts among men, the knowledge of which is acquired through bit by bit experience. For it is experience that causeth our life to move forward by the skill we acquire, while want of experience subjects us to the effects of chance. -Plato

Totally subjective question. Too many possible answers, and too many opinions as to what the baselines should be. For some it would be when your confidence as a player is validated by rarely if ever being defeated, For others it might be visiting every location and completing all the scripted questlines. Others might say, when the game becomes "comfortable to play" ie you can navigate effortlessly through all the menus / use hotkeys proficiently on the controller etc. Ultimatley,knowledge is power, the more you learn and use successfully within the game, the more your sense of mastery increases.