Mark, your wife can apply for an extension as long as it hasn't reached the expiration date of the current visa. The earliest she can apply is 28 days before her 30th month here in the UK. From that date till before the actual expiration date, your wife can apply for the extension visa.But of course, it's wiser to apply as early as you can to avoid further hassles.
A lot is actually confused about this 28 days before thing. It doesn't mean that it's the only time they can apply for a visa. It's just getting the date when they can actually start applying. And as long as the current visa hasn't reached its expiry date yet, it's fine to apply and used those valid dates in case there are more docs needed to get.