The 6 weeks they told you was probably referring to the cycling process. Tell us the specs of your tank and water paramaters and also how long your tank has been set up for.
Crabs do not like any by-products of the nitrogen cyclein their water, this includes nitrite, ammonia and nitrate (which fish can handle in larger quantities). For this reason I would say 3-4 months depending on the specific tank, 3-4 months in your case would be the minimum I would recommend.
Please also be aware that crabs need somewhere to leave the water and because they are amazingly good at escaping, this has to be planned very well. This means a tight fitting hood and at least 5 inches of clearance between the ground and the top of the glass with no cables or equipment that can be used as ladders. Think carefully