Hi Green,
It a good question ! I worked with other SIM solution and got exactly the same need. We can t determine number of log because it depends of a lot of factors ( Number of appliance , kind of device , number of users , behaviors of users , kind of logs you need to keep ...).
If it s a Juniper security environment and you manage everything with NSM, there is a log counter script, in order to do this Job. If it s not, you can forward every logs devices to a syslog server and do a wc -l in your log file everyday : That s what i did and it works pretty well.