Mash conversions take place in as little as 20 minutes. You can use tincture of iodine to test for starch. Place a sample of wort on a white plate and place drop of TOI into the puddle of wort. If it turns purple/black, there is still starch, and the conversion is not complete. Even with a dark stout, a puddle on a white ceramic plate should work with TOI. A 60 min mash is standard, because thats how our grandpappys grandpappy used to do it wayyyyy back in the day. Today, it may or probably may not be needed. I cannot see a 90 minute mash being needed unless it would be a traditional step mash (are undermodified malts even still available?). Imagine if you could save 30-60 minutes of your brew day, by mashing only as long as you need to, as indicated by the Tincture of Iodine. After the conversion, holding the mash serves no purpose.