If a person is pre-approved for a mortgage loan, is the rate quoted 'locked in' for a certain period

A pre-approval has NOTHING to do with locking a rate. Rates typically cannot be locked until a particular property has been identified and closing date set. Some lenders will do short term “lock and shop” loans, typically charging a cost to cover hedging expenses in case they don’t end up with the loan. Since costs vary based on lock length (30 day rates are better than 60 day rates) it wouldn’t be productive to lock a rate without an accepted sales contract and closing date, even if it were possible.