If I have to tell my insurer in advance, how much notice will I need to give?

In order to allow sufficient time to process documents, you will need to give your insurer sufficient notice of your intention to travel in advance. So, it is best to contact them about one month in advance.  

It is a requirement that your Green Card document covers a period of a minimum of 15 days. Therefore, if your motor insurance policy is due to expire less than 15 days after the date of travel, you will need to ensure you have confirmed your renewal before you depart (even if your trip is less than 15 days in total).  

As is the case with other forms of travel documentation, you may also need to allow additional time if you are travelling at a busy period, such as during the summer or around a Bank Holiday.