Formulas effectively include the formula's text they reference literally in the underlying SQL with some overhead. You need to simplify your formulas, or leverage workflow rules or Apex Code to minimize the impact on formula space, which is limited to 5,000 characters compiled.
As an aside, just like in algebra, you can pull common factors out of the case statement, leaving you with:
(IF(ISBLANK(Start_date_for_Remaining_Formula__c),NULL,
DATEVALUE( Start_date_for_Remaining_Formula__c ) +
CASE( MOD( DATEVALUE( Start_date_for_Remaining_Formula__c ) - DATE( 1900, 1, 7 ), 7 ),
0, 1, 4, 2, 5, 2, 6, 2, 0)
+ Time_to_Discount__c)
- DATEVALUE(NOW()))*24
Edit: Explaination
To arrive at the solution, I first pulled out the DATEVALUE(Start_Date_for_Remaining_Formula__c):
(IF(ISBLANK(Start_date_for_Remaining_Formula__c),NULL,
DATEVALUE(Start_date_for_remaining_formula__c) +
CASE( MOD( DATEVALUE( Start_date_for_Remaining_Formula__c ) - DATE( 1900, 1, 7 ), 7 ),
0, DATEVALUE( Start_date_for_Remaining_Formula__c ) + 1 + Time_to_discount__c,
4, DATEVALUE( Start_date_for_Remaining_Formula__c ) + 2 + Time_to_discount__c,
5, DATEVALUE( Start_date_for_Remaining_Formula__c ) + 2 + Time_to_discount__c,
6, DATEVALUE( Start_date_for_Remaining_Formula__c ) + 2 + Time_to_discount__c,
DATEVALUE( Start_date_for_Remaining_Formula__c ) + Time_to_discount__c
)
)
- DATEVALUE(NOW()))*24
Next, I noted that I could remove Time_to_discount__c, except that the "default" parameter would then have no value, so I substituted "0 + Time_to_discount__c" into that section:
(IF(ISBLANK(Start_date_for_Remaining_Formula__c),NULL,
DATEVALUE(Start_date_for_remaining_formula__c) +
CASE( MOD( DATEVALUE( Start_date_for_Remaining_Formula__c ) - DATE( 1900, 1, 7 ), 7 ),
0, 1 + Time_to_discount__c,
4, 2 + Time_to_discount__c,
5, 2 + Time_to_discount__c,
6, 2 + Time_to_discount__c,
0 + Time_to_discount__c
)
)
- DATEVALUE(NOW()))*24
Finally, you can move the "+ Time_to_discount__c" to the area after the case statement:
(IF(ISBLANK(Start_date_for_Remaining_Formula__c),NULL,
DATEVALUE(Start_date_for_remaining_formula__c) +
CASE( MOD( DATEVALUE( Start_date_for_Remaining_Formula__c ) - DATE( 1900, 1, 7 ), 7 ),
0, 1 + Time_to_discount__c,
4, 2 + Time_to_discount__c,
5, 2 + Time_to_discount__c,
6, 2 + Time_to_discount__c,
0 + Time_to_discount__c
) + Time_to_discount__c
)
- DATEVALUE(NOW()))*24
This leads to the final form I originally provided.