I usually set a separate file for trace logs.
file ike-trace.log size 5m files 5;
The "flag general" should be enough to give you basic information about IKE events without filling up the logs with a bunch of stuff to sift through.
You can bump it up to "flag all" if you need more information, or even throw in the hidden "set level 15" if you're feeling especially adventurous.
You can view the log with "show log ike-trace.log" from operational mode.