How long does it take to get 99 in a skill on runescape?

No one knows for sure how long it will take to get level 99, the only fact that is known is that when your level 91 in the skill, it is like starting from level 1 again back up to 91, to reach that 13m exp. There is no way of telling how long this take, not to mention it depending on your current skill level.

Note: This is a catch-all question, since this is being asked frequently.