It's depends on your wife's menstrual cycle..
Find the shortest cycle in your past record.
Subtract 18 from the total number of days in that cycle.
Count that number from day 1 of your current cycle, and mark that day with an X. (Include day 1 when you count.)
The day marked X is your first fertile day.
Find the longest cycle in your record.
Subtract 11 from the total number of days in that cycle.
Count that number from day 1 (the first day of your period) of your current cycle, and mark that day with an X. (Include day 1 when you count.)
The day marked X is your last fertile day.
Next Steps
For example: if your shortest cycle is 26 days long, subtract 18 from 26 — you get 8. Then, count 8 days starting from day 1 (the first day of your period). If day 1 was on the 4th of the month, you’ll mark X on the 11th. So the 11th is your first fertile day of this cycle — you should stop having vaginal sex on this day or start using another method of birth control
For example, if your longest cycle is 30 days long, subtract 11 from 30 — you get 19. Then, count 19 days starting from day 1. If day 1 was on the 4th of the month, you’ll mark X on the 22nd. So the 22nd is your last fertile day of this cycle — you can start having unprotected sex the next day.