in SharePoint 2010 ULS logging has been improved by reducing log file size significantly. This allows a new approach to setting log file retention policy. The default setting is now at number of days. So we can specify directly for how many days we want to keep the log files. The default setting for this is 14. Also a new option exists to restrict trace logs to a fixed disk size. This is unchecked by default. With this option we can define how much disk space we want to assign to the logs.
As an example On 15th Day, log files from day 1 are pruned. If restrict trace log setting is also enforced to a fixed disk size, then both settings are enforced and the order on which the settings are enforced is dependent upon which condition is hit first. So if we have set 5 GB for these logs and this limit is reached on 10th day then the log files are cleaned to satisfy these settings. If we hit the 15 th day without consuming the log disk space still the files are cleaned.