In our dojo we have time requirements but the instructors watch the students and if someone is ready sooner then they are asked to test sooner. If they are not ready then they need to wait longer and keep working. I try to give input on a regular basis as well. If a student asks to test they will be informed as to weather or not they are really ready but I find it disrespectful. I look at it as the sensei's responsibility to decide. I know an instructor that lets his students wait until they are way past ready and that his his decision. It doesn't need to be all about rank but what you learn.