How long is a CA Statement valid as per UKIV for UK visit visa purposes?

In the link kindly provided by @Gayot Fow is mention of:

eg bank statements or payslips from the last 6 months

What you are asking about is neither a bank statement nor a payslip but the above does give some indication of how up-to-date the ECO expects financial records to be, with 40 days being well within a reasonable time frame.

The Guide to supporting documents: visiting the UK specifically discounts some financial information, unless specifically requested, more than a year old:

bank statements or letters issued more than 1 year before the date of application

Bank statements and payslips are somewhat special cases where provision of a series at monthly intervals may be appropriate better to show a general pattern than just a snapshot that might not be representative. For example, a bank statement more than 6 months old may be the most practical way to demonstrate participation in a bonus scheme that only pays out twice a year.

If your Chartered Accountant's statement is important for your application (and note @Gayot's Comment They don't care about chartered accountant statements. It's doubtful anyone will look at it.) then I'd suggest anything up to 180 days is as good as anything more recent. After that, maybe request a new statement.