Hello,
the usage quota uses intermediary deltas to record changes which need to be collapsed before the current actual usage is visible for a user. This is done by the UserUsageCollapseJob which by default runs every 5 minutes. You can modify the schedule by editing the cron expression for the "userUsageCollapseJob" bean. Have a look at the scheduled-jobs-context.xml to find it and if you make modifications, please do it in a copy of the file that you put into /tomcat/shared/classes/alfresco/extension
Regards
Axel