rsync problem

[paul-lkw]

Hi all:
Recently I used rsync to backup my data via NFS, I use the options as below;

rsync -arclpog --delete --ignore-errors /source_path /NFS_remote_path

then in first time I backup all data for about 5 hours.
In theory it should be shorter in thereafter because rsync will just check the time stamp for update but finally I still get every time 5 hours to backup.
Do I used wrong option with this result?
Thx for all your helps.

^_^