Spouse Visa - IHS & Financial Requirement

I obtained my Citizenship on 10 years long route. My wife joined me in Sep 2012 as dependent when I was on T2 General. She still has the visa till May 2016 as T2 PBS dependent. Since then I got my ILR and the British passport couple of months ago. I understand that to extend her stay I need to use FLR(M) and i meet the minimum threshold earning. I want to know if I can apply for her extension to stay now? I have six months pay slips and bank statement ready. Do I need to maintain any balance for the same period?
Thanks in advance for reply.

She can apply now.

No maintenance balance required.