In the case of holidays it could be after just 10 days. In our Borough they will issue warning letters EXCEPT where it is an unauthorised holiday, then they may skip warning letters and go straight for FPN.
For example we put through 2 FPNs last week who had no warning letter, but were told before the holiday that it was unauthorised. They took the holiday anyway, so now we have requested the Attendance Improvement Officer to issue the FPNs. In one case the holiday was for 11 days. (Before you feel too sorry for that parent, the holiday was refused because the child already had an attendance of below 70%)
In the case of general absence then, yes, you will get a warning that your attendance will be monitored for the next 20 school days and that ANY further unauthorised absence in that time will result in an FPN. This is usually the case where a parent is just not bothering to send a child to school and is mostly unconncected to holidays.