the sooner (,) the better

I have a question about "the ... the ... " type of sentences: should we put a comma in between the two parts?

I would write a comma, but a colleague of mine said you should not do that. Indeed, I have notice that "the sooner the better" is not always written with comma, but other longer sentences contain a comma:

The longer you keep this wine, the better it tastes
The better your sentence, the easier it is to punctuate.

Can you clarify this point for me? Thanks

[Curious Cat]

Longer sentences need commas because each of them has its own subject-predicate set, that's why they should be separated. If "the sooner the better" is used indpendently, to me as a non-native speaker it looks better without any commas I'm not a grammarian, so please don't consider it as a rule

[Casiopea]

The general rule of thumb is to use a comma to set off a phrase or clause that expresses a contrast. For example,

[1] The sooner we get her to the hospital, the better off she'll be.
[2] The sooner, the better.

Note, if the phrase or clause is short and part of the flow of the sentence, then a comma is often not used:

[3] The sooner the better.