Rank requirements are basically 30 hours of - quality - mat time for colored belt levels, and one year from Sankyu through Ikkyu, before testing for Shodan. However, this is really dependent on:

• quality training time and 
• a steady frequency of attendance. 

It is generally assumed, and practice has proven this out, that it takes from 5 to 6 years to reach black belt proficiency, student and teacher quality dependent. There are exceptions to all rules, and some students could exceed these times or even improve on the periods. The guidelines are suggestions. The real measuring stick is quality and proficiency.  In other words, "standing on the mat edge for two years does not a Nidan make.