Interesting question. Tórshavn (in the Faeroe Islands) to Seyðisfjörður (in Iceland) takes about 16 hours.

Tórshavn to Esbjerg (in Denmark) takes about 38 hours.

When there was a crossing from Hantsholm (also in Denmark) to Seyðisfjörður without stopping off at the Faeroes on the way, that took about 50 hours.

The Smyril Line runs these sailings — it is a Faeroes-based company.

I remember some years ago getting the ferry from Lerwick (on Shetland) to Aberdeen. I was rather startled to see that the timing was given as 17—34 hours, depending on the seas and weather. (I wouldn't want to be on one of the 34-hour crossings...)