The question raised by the original poster is a staple of this newsgroup so let me add a new slant and say perhaps the answer should be never. Or to put it another way the question is meaningless...

Consider our rookie arriving fresh scrubbed in his first poker room; money in one pocket, a well thumbed copy of Lee Jones in his other.

He considers all the games available in the room and selects, naturally, one at the lowest level.  He may win or lose the first few times.  But with experience, and study he gradually improves.

He also starts to get to know the regulars.  He begins to know who the winners and losers are.  He also gets some experience against players who normally play at higher levels but, for whatever reason, are passing time in his game.

One day he comes to the poker room, feeling good, and with a healthy backroll.  This time he sees a game at the next higher level.  In it are a few players he's not seen before; a few who are regulars but he has played against in the lower game and doesn't fear; a few who only play at the higher level but he knows are regular losers; and perhaps just 1 or 2 of the tougher players who he rightly respects.

So he gives that game a try.  The weaker players in the game protect his ev and he gives respect where due, watching the table experts and learning from what they do and how they play.  

The next few times he plays there may be no game at that level he feels comfortable with.  So he plays at his normal level.  But from time to time the right games present themselves and he takes the opportunities as they arrive.  He never actually moves to a higher level.  He continues to play at tables where he feels he has the advantage.  But gradually he finds that his normal game is at the higher level. Sometimes he will play in the lower game; sometimes he will spot a game
at an even higher level which he knows he can beat.

So, my long-winded point is, that a player should never conciously move up from one level to another.  Instead he should consider all the available games and select the one which offers the best advantage.  If a player is working hard at the game then gradually this 'best' game will be at increasingly higher levels - but every time he plays he concentrates on game selection, *NOT* level selection.