When I am writing a book and referring to something that lasts 2-8 days, do I say "2-8 days" or "two to eight days"? Also, when referring to how much of something I would use, would I say "2-4 tablespoons" or "two to four tablespoons"?
Both are valid, but imho the numerical format is more succinct and stands out from the surrounding text.
Consider this sentence:
During my holiday trip, I will spend just 1-2 nights in London followed by 10-15 days touring the rest of Europe, and return by 28th or 29th.
Or some (made up) recipe:
1-2 finely chopped onions
3-5 tsp flour
Few pinches of salt
In both these, I think writing out the numbers in long hand will make it excessively verbose.
As a counterexample, consider the lumber size 'two by four' , representing it as '2 x 4' may take a bit of processing for people who parse 'x' symbol as 'into/multiplied by'. However, when a smaller number is followed by a larger number and separated by '-', it's common to read the symbol as 'to' (unless it's in a mathematical sense).
Both convey the same meaning but the fully written form "two to eight days"/"two to four tablespoons" doesn't leave place to any effort of interpretation: It explains what the dash would stand for.