If you want to know this to figure out how much Redo is generated every day (or hour) then the best bet will be to look into the Alert file for all the Normal Log switches (switches that were not caused manually)

Count the number for a reasonably long period (a week or a month ) and then devide by the number of days in that peiod to get a rough idea of the redo amount per day.

For accurate results, this assumes that the instance was not shutdown during that period.

- Rajeev