1.Create a field for business days using a number formula field. Use the following formula to calculate minus weekends.

CASE( 
MOD(DATEVALUE(CreatedDate) - DATE(1900, 1, 7), 7), 
0, (TODAY() - DATEVALUE(CreatedDate)) - 1 - FLOOR((TODAY() - DATEVALUE(CreatedDate))/7)*2, 
1, (TODAY() - DATEVALUE(CreatedDate)) - FLOOR((TODAY() - DATEVALUE(CreatedDate))/7)*2, 
2, (TODAY() - DATEVALUE(CreatedDate)) - FLOOR((TODAY() - DATEVALUE(CreatedDate))/7)*2, 
3, (TODAY() - DATEVALUE(CreatedDate)) - FLOOR((TODAY() - DATEVALUE(CreatedDate))/7)*2, 
4, (TODAY() - DATEVALUE(CreatedDate)) - FLOOR((TODAY() - DATEVALUE(CreatedDate))/7)*2, 
5, (TODAY() - DATEVALUE(CreatedDate)) - 2 - FLOOR((TODAY() - DATEVALUE(CreatedDate))/7)*2, 
6, (TODAY() - DATEVALUE(CreatedDate)) - 2 - FLOOR((TODAY() - DATEVALUE(CreatedDate))/7)*2, 
null)

2.Create a date/time field for the current date/time 

3.Create workflow rule to update this field every time it is edited with formula below

Now()

4.Create a number formula field to calculate the age difference of the case today. I used the formula

VALUE( MID( TEXT( NOW__c ), 12, 2 ) ) - VALUE( MID( TEXT( CreatedDate ), 12, 2 ) )

5.Create a number formula field for the business hours of the company ie. 8 hrs

6.Create a number formula field to calculate the total age of the case in business hours

(business_days__c * business_hours ) + age_difference_of_case