But as far as I can recall, we haven't discussed relevance conditionals, as (I think) exemplified in this morning's Stone Soup:
Also known as "conditional speech acts", these are covered in section 5, Factual and relevance conditionals, of Rajesh Bhatt and Roumyana Pancheva's "Conditionals" (chapter 16 in the Blackwell Companion to Syntax). They give these examples:
If I may be honest, you are not looking good.
If you want to know, 4 isn't a prime number.
If you are thirsty, there is beer in the fridge.
They explain that "The if-clause in relevance conditionals specifies the circumstances in which the consequent is discourse-relevant, not the circumstances in which it is true."
Though on reflection, the example in the cartoon is not quite like these others. As the term "conditional speech act" suggests, relevance conditionals can be understood as ordinary hypothetical conditionals in which the consequent includes an unexpressed verb of communicating, something like "If you're thirsty, I'll inform you that there's beer in the fridge".
Val's sentence seems to express a genuine hypothetical in a differently elliptical form. What's left out, it seems, is something about the hearer's uptake, not the speaker's output: "If you think about it, (you'll see that) that's not very long".
Because the "you'll see that" (or "you'll agree that" or whatever) is omitted, there's an opening for Holly's rejoinder, just as in the canonical relevance conditionals ("And if I'm not thirsty, does the beer magically disappear?").
Val's version of the sentence doesn't permit then, which Bhatt & Pancheva give as a characteristic of relevance conditionals:
*If you think about it, then that's not very long.
Adding "you'll see that" or similar material changes this:
If you think about it, then you'll see that that's not very long.
[Update — Kai von Fintel writes:
"Biscuit conditionals" as they're often called (after Austin's example) have been discussed quite a bit in the recent scholarly literature. In addition to the Fulda paper adduced by Joseph, here are some others:
DeRose, Keith and Richard E. Grandy. 1999. Conditional assertions and "biscuit" conditionals. Noûs 33(3): 405–420. doi:10.1111/0029-4624.00161.
Siegel, Muffy E.A. 2006. Biscuit conditionals: Quantification over potential literal acts. Linguistics and Philosophy 29(2): 167–203. doi:10.1007/s10988-006-0003-2.
Franke, Michael. 2007. The pragmatics of biscuit conditionals. ms, Universiteit van Amsterdam.
Predelli, Stefano. 2009. Towards a semantics for biscuit conditionals. Philosophical Studies 142(3): 293–305. doi:10.1007/s11098-007-9187-8.