I think it depends on whether the problem lends itself to abstraction or not. If I have a problem that requires 1000 lines to solve several large system of linear equations, I'm absolutely going to be separating that out. On the other hand, if I have 1000 lines of "if in country A, apply this tax rate", then I'm not going to separate it, because there is no underlying abstraction to use.
That's actually one of the more reasonable uses for OO; have an interface that defines 'apply_tax_rate', and a bunch of implementations for each, so you only have a single line in your function (instance).apply_tax_rate. Of course, that leads to your actual implementing code being scattered in different classes, and that can be a pain, too.
If a functional language, I'd prefer each being its own function, with the case matching in a single, standalone file that returns the appropriate function based on whatever. That way you still get just apply_tax_rate(type, amount) in your main calculation function.
