How many years does Sm Entertainment Artists become trainees??

by looking for this chart, we can see some of SNSD members is the longest trainee in SM and joined first comparing some of other group member like Super Junior's or even TVXQ's one.. Ironically TVXQ and Super Junior and even The Grace (CSJH) has debuted quicker than SNSD.

TVXQ (2003):
Max Changmin - 1 years
Hero Jaejoong - 2 yearsU-know Yunho - 3 yearsXiah Junsu - 4 years

Super Junior (2005):
Kyuhyun - -1 year
Shindong - -1 year
Ryeowook - 1 year
Kangin - 2 years
Siwon - 3 years
Heechul - 3 years
Kibum - 3 years
Sungmin - 4 years
Hangeng - 4 years
Donghae - 4 years
Yesung - 4 years
Leeteuk - 5 years
The Grace (2005):
Stephanie - 1 year
Sunday - 3 years
Lina - 3 years
Dana - 4 years
SNSD (2007):
Sunny - -1 year
Taeyeon - 3 years
Tiffany - 3 years
Seohyun - 4 years
Yoona - 5 years
Yuri - 6 years
Jessica - 7 years
Hyoyeon - 7 years
Sooyoung - 7 years
SHINee (2008):
Onew - 2 years
Minho - 2 years
Taemin - 3 years
Key - 3 years
Jonghyun - 3 years

f(x) (2009):
Amber - 1 year
Victoria - 2 years
Krystal - 3 years
Luna - 3 years
Sulli - 4 years

blue: Has been trainee for two years or less until debut.
red: Has been trainee for five years or more until debut.