When an employee has been terminated from employment, how many days after termination does the employer have to pay the insurance premium on the employee while waiting for the employee to decide on COBRA?

They don't pay while you are deciding on COBRA. You would be termed from the plan in accordance with company policy. Usually the day of term or in rare cases at the end of the month termed.
1) The employer has up yo 45 days to notify you of your COBRA options.
2) From the date of notification you have 60 days to decide.
3) Then you have up to 45 days to make your first payment. Which is all the premiums due, retroactive back to the day you were termed.
4) Once they receive your payment they will enroll you retroactive to the date of term into the health plan.
That is for Federal COBRA which is for businesses with more than 20 employees. Employers with 2-19 would be under state "Mini COBRA" regulations if they have them.