How to create user defined function for below scenario?

Question

I have to create user defined function.

That will insert data in No_of_work_period column.

On below scenario.

i)It should insert Number of months in 'No_of_work_period' column if Salary_Frequency is Monthly.

ii)It should insert Number of weeks in 'No_of_work_period' column if Salary_Frequency is Weekly.

These user defined function will be called in stored procdure from their it will update data in column No_of_work_period.

I hope you guys understand my problem thank you.