Using Log4Net 1.2.13 we use the following configuration settings to allow date time in the file name.
<file type="log4net.Util.PatternString" value="E:/logname-%utcdate{yyyy-MM-dd}.txt" />
Which will provide files in the following convention: logname-2015-04-17.txt
With this it's usually best to have the following to ensure you're holding 1 log per day.
<rollingStyle value="Date" />
<datePattern value="yyyyMMdd" />
If size of file is a concern the following allows 500 files of 5MB in size until a new day spawns. CountDirection allows Ascending or Descending numbering of files which are no longer current.
<maxSizeRollBackups value="500" />
<maximumFileSize value="5MB" />
<rollingStyle value="Composite" />
<datePattern value="yyyyMMdd" />
<CountDirection value="1"/>
<staticLogFileName value="true" />