I suppose it depends on how well balanced the cells/subpacks in the battery are. If they're new and have similar self-discharge rates, then you could leave it sitting for a much longer time before it caused any problems, since the car itself would re-charge the subpacks to a similar level on its own when you resume driving.
The problem is when the subpacks begin to drift away from one another in capacity & self-discharge rates. (Like this car's old pack. Have a look at my refurbishing data spreadsheet: Columns W & X give you an idea of how much my subpacks self-discharge over 1 & 7 days respectively:
https://docs.google.com/spreadsheet/...hl=en_US#gid=0 )
I can't find the reference now, but I thought I read somewhere that Honda recommends starting/driving its hybrids at least once every month for 30 minutes. I suspect even that's too long for one with a tired pack.
Grid charger required for proper care & feeding of elderly packs.