Tier 2 general extension - how many months in advance?

[vikas.taurus]

Hi All,
May I please know how many months in advance can I apply for extension of Tier 2 General visa. I have more than 6 months left on my current visa. After searching all I could find about Cos was:-

"39.18 When you assign a CoS to extend a migrant’s leave in the same tier and category, the start date on the CoS should be the day after their current leave expires"

https://www.gov.uk/government/uploads/s ... _11-14.pdf

Does anyone have any idea if my sponsor can apply for a new CoS so that I can apply for a visa extension.

PS:- I want to extend soon as I will be applying for T-2 (dependent) visa for my fiance and she will get the visa which will be valid til my current visa is valid. That means that I will have to apply for another T-2 (dependent) visa in 6 months.