ILR M SET EARLIEST APPLICATION DATE

Hi guys

I'm slightly confused with ILR SET (M) n I'm hoping you might be able to guide me in the right direction?

my wife entered the UK on a spouse visa on 22/01/2015 (issued 31/12/2014) and her FLR expires on 30/03/2020. She is on the 5 year route.

Was wondering when is the earliest date she can apply for ILR?

I'll be grateful if you can point me towards the home office guidance too.

thanks.Taas

within 28 days of her completing 5 years, counting from the day she entered the UK