From the burgular help file, the following is written:
Master burglars begin to learn the path of the assassin, most of all. The art of critical strike against opponents (i.e., backstab) becomes available, as does the method of working with poisons and setting traps.
So my question is... when do one become a "master burglar", or a master of his/her profession?
After you've broken into Tektolnes' tower and stolen his water dish.
More seriously: You don't really know, until you get there. As with most things, expect it to take time and effort, and smart thinking. I suggest not focusing too much on your skills or branching them. It's a proven law of the MUDverse that your skills will only branch when you've given up trying.