5 minutes if you know CSS. And if you don’t, about the time for you to ask someone that knows CSS. In the worst case, the amount of hours to learn CSS.

So if you’re doing web pages, learn CSS.

Generally, if you’re doing something that directly involves X, learn how X works.

ADDENDUM

In most jobs, you’re going to be involved in only a few distinct technologies, learn those well and life is going to be easier. And most are transferable to the next job.