How long should someone stay a white belt?

depends on the style, the teacher, and the school or systems requirements. when i was a kid i started out in american freestyle karate and it took me about a month or two if i remember correctly to get past white belt after a few stripes. that time frame is common in alot of karate or taekwondo schools. now i take brazilian jiu jitsu at white belt all over again. from what i understand i can expect to be a white belt in bjj for a little under a year to two or more years depending on my ability to perform and retain knowledge.

Depends.

Sure, there are average time lengths, but as mentioned it takes as long as it takes.

I've seen sh it schools give their students their yellow belt after a few weeks. I've seen others that take 4 to 8 months. Other schools a year.

It all depends on the qualifications the instructor lays out.

It takes as long as it takes. We don't all learn at the same pace. We don't all put the same effort into getting better. Until you meet the standards to test for the next belt you should remain a white belt.

Until they meet the qualifications for the next belt. That varies from one person to the next and how fast they learn and how much effort they put into it. I would also make the exception for people who have altitude problems or mental problems where teaching them could end up a liability. Those people I would not promote either but they are rare.

In a good school, each student is allowed to progress at whatever speed is right for him or her. One indication of a poor school is preset rates of promotion, because we all come into this with unique physical and mental traits.

3 months. But I kept mine for two years and jumped belts. Not all schools will let you do that, but it helps to build humility, and lack of reverence for status (keeping up with Joneses).

Well for my school it would be until they have learned enough of the Basics and one Kata, walking and performing blocks and strikes, kicks... and how to count to 10 in Japanese... this is going to Vary style to style school to school too. To me until they are ready to test for the next rank requirements period, no time limits.

i agree 3 months