How long does it take to Ahref index my backlink?

Although it takes some time. We can say by Ahrefs runs bot or own web crawler, to retrieve information. In that the domain which are more likely will be crawled quickly where as the domain which are less popular will be crawled partially. In this manner they are working.

But you can check whether it is indexed or not by checking in regular google search by, site:your url.

The Best way to Check is Google search Like Site: Your URL

There is no exact time to index, will depend on quality