Jody,
The Singer 221-1 Featherweight manual says at the top of page 24:
"To insure easy running, the machine requires oiling and if used continuously it should be oiled each day."
The qualifying part of the sentence is; if used continuously. So, what that means is the more you use the machine, the more it needs oiling.
The next line says: "With moderate use, an occasional oiling is sufficient."
They don't define 'occasional' but I'd oil it twice a year if it didn't get used continuously.
Joe