Between one and four times the point size
A first-line indent is the most common way to signal the start of a new paragraph. The other common way is with space between paragraphs.
First-line indents and space between paragraphs have the same relationship as belts and suspenders. You only need one to get the job done. Using both is a mistake. If you use a first-line indent on a paragraph, don’t use space between. And vice versa.
But use your judgment—consider the width of the text block when setting the first-line indent. For instance, narrow text blocks (3″ or less) should have first-line indents toward the low end of this range. Wider text blocks should have bigger indents.