A long tail keyword typically has low search volume. give two reasons why a webmaster should not ignore long tail keywords.

It is probably a good idea to keep these long tail keywords in your inventory because if the number of search queries for these keywords increases by even a small amount, they might be reactivated and will start triggering your ads. They may also have strong quality scores so are worth keeping just for that reason alone.

BTW - Google’s system checks and updates the low search volume status once a week.