When to apply wife visa?

I have ILR and my wife have Tier 1 g main applicant visa valid until Feb 2016, she is completing 10 years in April 2018. We have one british national daughter and 2nd baby expected in Nov 2014. I need to switch my wife visa from tier 1 general to FLR(M). If we apply now then she require 2 extensions and if we apply in Feb 2016 then she only need one extension.

When my wife needs to apply new visa? either before 6 months expiry of current visa or she can switch in Feb 2016?

She can fulfill english language and financial requirements.

Please let me know.

kind regards.