Chris Webb runs a test:

The September release of Power BI saw the introduction of a powerful new feature: full text indexing, which makes searching and filtering on text columns much faster and enables the use of the new TextContains() and TextSimilarity() DAX functions. You need to enable it on a per-column basis and when you do that you have the option to rebuild the full-text index for those columns when you do a Full refresh or when you do a refresh of the type Indexes. So the big question is: how long does it take to build a full-text index on a column and what impact will it have on your overall refresh times?

Read on for that answer in a reasonably-sized dataset containing street addresses.