Validity of GTV dependent visas after main holder gets ILR & how many months prior to their expiry can they be extended?

I am considering applying a GTV dependent visa for my wife. My GTV BRP expires on 04 Sep 2023, and therefore I guess that her dependent GTV intial BRP shall also be issued to expire on that same date.

Now, the interesting situation. I intend to make a super priority ILR application in August 2022, and being a straightforward case, expect no significant delays and therefore, believe that my ILR shall be issued around mid-September 2022.

I understand that my wife's GTV dependent visa can be extended up to a maximum of 5 years (even after the main GTV holder has obtained ILR).

The question is: "how long before the expiry of her initial BRP can she submit her extension application as a GTV dependent?"

Can you please keep your questions in your existing topic!!! It is not necessary to start new topics asking the same questions!!