I assume that the application should create only one log file every time it runs, so you do not need a rolling file appender (though my solution would apply for rolling file appenders as well):
<appender name="FileAppender" type="log4net.Appender.FileAppender">
<file type="log4net.Util.PatternString" value="c:\temp\App-%date{yyyy-MM-dd_HH-mm-ss}.log" />
<appendToFile value="true" />
<layout type="log4net.Layout.PatternLayout">
<conversionPattern value="%date [%2thread] %-5level - %message%newline" />
</layout>
</appender>
(Obviously you can use other your own layout and other settings for the file appender.)