Be skeptical of using formula fields for datetime difference calculations because of a limitation with formula fields, quoted below:

> Important: Be aware of timezone conversion issues when using formula
> fields to calculate datetime differences:
> 
> Within the Salesforce application, dates & date/times are stored and
> managed in UTC (Universal Time Zone or Greenwich Mean Time). Date and
> Date/Time field values are converted and displayed in the timezone
> specified in your user personal information settings. **However, in a
> **formula**, NO TIMEZONE CONVERSION IS PERFORMED. This means, unless you happen to be in London, your date calculations will be off by the
> number of hours you are shifted away from GMT (and don't forget about
> daylight savings time).**
> Source: https://help.salesforce.com/articleView?id=000324878

If you are ok with those risks mentioned above, then you could see "Find the Number of Business Hours Between Two Date/Times" here https://developer.salesforce.com/docs/atlas.en-us.usefulFormulaFields.meta/usefulFormulaFields/formula_examples_dates.htm

As of January 2021 it seems that declarative Formulas (incl. flows/workflows/process builder, etc ) are limited in that they cannot access setup menu's concept of BusinessHours. Hardcoding workaround variables in a long-written formula is not a perfect solution for all user time zones, therefore until the GA Roadmap matures its declarative featureset, Apex is more accurate/durable at the moment for this question. Warning: Apex, if written synchronously can break UX if there's unhandled errors; If you're new to apex I'd suggest using async calls, if requirements allow for it. One option would be in a future call called appropriately from your trigger framework

An alternative is to perform the calculation using the BusinessHours object, such as possible with Apex development ie `BusinessHours.diff` (more on that in the comments)