{exp:channel:entries search:duration_field="<= 40 minutes"}?

Ah- that’s a clever idea, but I can’t think of a solid way to do it. The problem is, the duration field is a string. So even if you specified the time only in minutes (for example) 6 is going to be be bigger than 52. (If all your numbers were going to be 2 digits or something, you could do it. I’m assuming they aren’t. See here for details: https://www.copterlabs.com/natural-sorting-in-mysql/)

While I vastly prefer the duration field for display purposes, if searching/ordering by a number is critical, you’d need to go with say a text field set to ‘integer’ type.