Ask a Magician Thread.

There's no "specific level" sir.

Once you hit 4th job (for mages), the decision on where you can train is highly dependent on how fast you can kill the monsters. Lets put it this way: Start going when you start 2-hitting Skeles. (Should be slightly earlier than AM's due to the fact you have the Holy elemental advantage)

Personally I would be fine with 3-hitting Skeles, but then again I've never played a 4th job mage, so....

It should be added that fresh 4th job mages tend to die a lot at skeles, according to my experience as a LOLPALLY. It's either they get into the groove move with training (to prevent stupid deaths), or they bunker up more MP before nuking Skeles for exp.