When a tape is overwritren everything on the tape is erased, hence the actual date of when it can be overwritten keeps moving backwards depending on the last time a data write occured. In other words you have to add your overwrite pritection period to the time of the end of the last job that uses the tape to work out when the tape can be overwriiten.
Unlike a disk (parallel access devcie) tapes are serial access devices so you cannot only erase/overwrite one backup set on a tape it has to be all of them. Which is where append timings come in, as an append is an operation to add data to unused space on the end of a tape. The timings for append period do not move from the point that the specific tape is overwritten as all they do is lock down how long the append option is available from the first overwrite.
Assuming you only write data to each tape daily tape on one night then to re-use it them the same day the next week, the overwrite protection will need setting to 6 days (Not 7) Likewise the weekly ones would need to be protected for 27 days (to allow for February)
If you write multiple servers to the tape, it will need an append period so I would suggest maybe 12hours (assuming your job finishes overnight) might be enough.
