When should I start farming Crawmerax minions for efficient leveling?

The best point to start doing that is when you can kill the spawns with only a few hits (less than 20 or so usually). In order to achieve this, you have to know their elemental weaknesses.

The green ones are weak to fire. The fast blue worms are weak to electric, and the armored ones are weak to acid.

To get some good elemental weapons, try farming the last mission of the Armory DLC, there is a glitch you can do to get unlimited armory time, as far as I know its still possible to do it. If you don't want to exploit the glitch, then you can do the missions that give you a second and third crack at getting something decent in the armory. (or you can go online and try to find a crawmerax farm to get a good weapon). Keep in mind anything crawmerax drops will be very high level anyway.

Honestly, I think the Eridian Promontory is the best place to get XP. The Eridian aliens give up a tremendous amount of XP, and if you have some good static weapons (or if you are a hunter with trespass maxed out) they are relatively easy. At the very least, I'd say that for gaining XP, they are certainly more effective than dealing with Crawmerax and his minions.

UPDATE: The "non-DLC" content will NOT be under-leveled provided you complete the mission "Find Steele" (on playthrough 2), and your level is at least 49. This is because ALL enemies will scale with you, and hands down the Eridian Promontory is the easiest place to farm once this happens. They will be about 2 levels above you and they give far more experience than equal level creatures of a different type. Please follow the link for more information on enemy level scaling.