conditionally create and save separate log file in php/symfony2
i think you must create a custom handler.
With Monolog, you can log in a database (see for example https://github.com/Seldaek/monolog/blob/master/doc/04-extending.md)
Thus, it's easy to know how many times an error was raised since x days.(something like : "select count(*) from monolog where channel='...' and time>...")