Maybe like this
set the workflow as scheduled to run at regular intervals like you want
Use a condition with a formula to check if the current time is between 2:00 pm and 6:00 pm
Use formula like : TO_NUMBER(TO_CHAR(SYSDATE, 'HH24')) >= 14 AND TO_NUMBER(TO_CHAR(SYSDATE, 'HH24')) < 18
Then workflow actions are executed only if this condition is true.