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Every few months is fine. You want something from the very start of the period and then something towards the end.

How about the signed letter from my parents saying that we have been paying rent since November 18th 2018 until now.
 
How about the signed letter from my parents saying that we have been paying rent since November 18th 2018 until now.

You can include that but there is no proof that that is actually the case. Parents aren’t always great people to vouch for their children.
 
You can include that but there is no proof that that is actually the case. Parents aren’t always great people to vouch for their children.

But I thought that it's good to include that kind of letter when applying for the actual visa if you haven't got a tenant agreement.
 
But I thought that it's good to include that kind of letter when applying for the actual visa if you haven't got a tenant agreement.
It's additional material, but it doesn't ever proof cohabitation because parents will be biased.