I'm not sure exactly what you mean. What most people think of as Tomcat's main "log" file (catalina.out) isn't even a proper log. It's just the redirected stdout/stderr from the JVM that Tomcat is running in. And, incidentally, because it's not a true logfile, it's not recommended for webapps to write to stdout/stderr.
Are you referring to one of the other logs such as the localhost log or the log produced optionally when the request-tracking valve is enabled (access log)?
An IDE is no substitute for an Intelligent Developer.
Actually, unless you edited your topic title, I was suffering from my usual early-morning blindness. At any rate, it says very specifically "Access Log Valve" now.
I can't think of any way to specify a pattern using their rules that would be good for every 2 or 3 or whatever hours. You can always check to see if there's a crontab-like pattern option attribute. Otherwise, your best bet is probably to yank a copy of the accesslog valve code and create a customized copy.