Consider using **Apex** instead of formula. Here's why:

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 those risks mentioned above are acceptable, then you could check out "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 (ie `$Setup.DefaultBusinessHours` doesnt exist and will not compile/save into a formula). 

Many google results settle for a non-perfect solution by using long-written formulas that hardcode/assume one timezone and working hours, but they often dont control for holidays and are not a perfect solution for all user time zones. Formulas have strict length limits too so you cannot easily write formulas that handle for all those realities. Therefore until the GA Roadmap matures its declarative featureset, Apex is more accurate/durable at the moment for this question. 

If anyone reading this is solving or experimenting with Apex, then involve a developer and follow best practices.  For example: if written poorly in synchronous context you could potentially break ux for other users ie if theres any unhandled errors; So to anyone new to Apex I'd suggest using async calls, if requirements allow for it.  One option of many options would be in a future call called appropriately from your trigger framework.  The main logic could perhaps call `BusinessHours.diff`