If there is a one to one relationship between these two tables, this should work. If you have more than one record in the second table (where you defined the duration field), then you may not be referring to the record you thought you were in the calculation.
If I have this relationship:
Tasks::ID = DurationTable::ID
and my current record in the "tasks" table has ID = 1
And I have these records in "duration":
Then Table::Duration * 7 will evaluate to 35 for the current record. The records with durations of 6 and 8 cannot be accessed by this relationship.