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Your wife’s BRP card will state what date the spousal visa expires. It’ll be valid for 33 months so if it was issued in December 2016, it’ll probably expire around August 2019.
The next visa will be FLR(M), which is valid for another 2.5 years and then she can apply for ILR after 5 years in the UK. Once she has ILR she can qualify for UK citizenship.
So, for the next visa, she can apply for it up to 28 days before she reaches 30 months in the UK... so it will depend what date she actually arrived in the UK as to when she can apply.
Assuming she arrived in December 2016, she will reach 30 months in June 2019 and so can apply for FLR(M) 28 days before that.
The FLR(M) has the same requirements as the spousal visa (financial, accommodation and relationship), but instead of proving communication and visits to each other, you provide evidence that you have been living together at the same address in the UK for the last 2 years leading up to the application (I.e. May 2017 to May 2019).
You do this by providing 6 official documents in each name from at least 3 different sources spread evenly over the 2 years (every 4 months). They can either be jointly addressed or separate or a combination of both. You can use things like utility bills, tenancy agreements, bank statements, letters from the NHS/DVLA/HMRC etc.
So if you haven’t already, you should get her name on as much mail as you can, switch to paper statements (don’t go paperless) and also save EVERYTHING.
For more detail, see the FLR(M) form:
https://www.gov.uk/government/publications/apply-to-extend-stay-in-the-uk-as-a-partner-or-dependent-child-form-flrmSent from my iPhone using Tapatalk