On TVTropes, Webcomic Time (link) is when real-world time does not reflect the passage of time in the comic, basically, real-time and time in the comic book are asynchronous.
On the link above, it said that one webcomic had time running at a $12\colon1$ ratio of time, so 1 month in the webcomic is about a year in real life.
I tried this calculation:
$$12 + 1 = 13$$
to find out the parts.
How would I find out what $3$ months in the webcomic is equivalent to, in real life, or even for $18$ months - what would be the correct calculations?
I would appreciate any help. I've been learning about ratios recently, and want to ensure I get the calculations 'right'.
Your concept is not so accurate for this scenario. What they meant is that $$\frac{\text{a unit time in webcomic}}{\text{a unit time in real life}}=\frac{12}{1}\text{.}$$
This means that you simply have to multiply the time in webcomic by $12$ to get an equivalent time in real life.