According to the post, the idols who took the shortest time to reach #1 were:

• CN BLUE: 15 days after their debut with “I Am a Loner“
• miss A: 22 days after their debut with “Bad Girl Good Girl“
• 2NE1: 40 days after their debut with “Fire“
• F.T. Island: 55 days after their debut with “Lovesick“

On the other hand, the idols/singers who took the longest time to reach #1 were:

• K.Will: 1,490 days after his debut with “My heart is Beating“
• Jewelry: 1,494 days after their debut with “Superstar“
• IU: 818 days after her debut with “Good Day“
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• [Note] WOW! I'm pretty amaze that CNBLUE took 15 days after debut to win #1~I love I'm a loner.. I remember when it came out, i used to listen to it everyday, and i still do..This shows that this song is Awesome! 2NE1 and Miss A~ No suprise, hence that they are really good ><
• @Allkpop