Hi StefanWilmes,  It seems you are not the first to run into this issue. The workaround has been to use "isi_classic" commands to set the duration.  You may want to consider opening a service request so that a ticket can be logged for frequency within Isilon Support.  [URL]
When we create a SnapshotIQ schedule with a specific duration, OneFS adds 1 to the duration. For example, if we create a expiration of 3 hours, after creation of the schedule, reveals a expiration time of 4 hours. When setting the expiration time to 1 weeks, isilon sets the the expiration time to 8 days It doens't matter if you use the GUI or CLI.. We are using OneFS 8.0.0.1. Anyone seen the behaviour and has a solution to this problem ? 
SnapshotIQ expiration error/question