Time to put a new Tier- 2 General application- Change of Emp

[monushh]

Dear Gurus

I have query about the time a Tier 2 General visa holder needs to wait to put a new Tier 2 General application(Change of Employers).

My wife is currently on Tier 2 General visa which was approved in February 2017 and this expires in March 2018.

She has got a new offer where the new employer has issued her with a COS in which the start date of the new employment states as September 2017 to August 2019.

As the start date is in September she is planning to put the application towards July this year.

Therefore, please can you advise if she can apply for a new Tier -2 General visa as her last visa was approved recently i.e Feb 2017?

Is there a timescale/ wait she needs to obey before putting a new application?

Kindly advise.

Thank you.