Hello Mahoori
you can refresh your snapshot through Enterprise Manager.What u have to do is make your shedule with a time of 8 hrs so that after every 8 hrs your snapshot will be automatically refreshed.How u configured your snapshot whether it is manual or automatic,better do it automatic with a time gap,since i configured my replication with a interval of 24 hrs so that all the updates will be refreshed after working hours this i did it because of our network is very slow.When u decide how many times your snapshot to be refreshed it depends mainly on your network.
All the best
regards
akhadar