How long can a Renault ZE be left, and still start?
FWIW - I recently left my Fluence for 3 full weeks, and it started without any indication of a flattened 12V battery. The traction battery was left at half-way indicating in the second 8th sector (~20% charge) and it had not noticeably changed at all after the 3 wk period.
The performance of the 12V battery seems to be a potential 'weakness' for EVs in that without it you won't be able to activate the traction battery relay that will then recharge the 12V battery. However, unlike an ICE car where the decline of a battery is gradual and it is clear when the 12V battery is giving up the ghost (by an increasingly problematic struggle to turn the engine over), for EVs it will either work, or not. So it might come as a rather unexpected, and frustrating, surprise if one discovers the 12V battery can't energise the battery relay. I was somewhat worrying that this would be the case after 3 weeks (given what I had heard of Leafies), but the Fluence was fine.
There is a comment in the owners book to ensure the battery is replaced after a certain time, irrespective of any indication of problems. I think that is very wasteful indeed, if it is unnecessary - my 13 yr old Vectra diesel has its original battery* and still cranks over perfectly - although it is beginning to struggle if it is left for a week or two in winter now, so is slowly approaching a need to be changed.
*(used daily, and benefits from a padded blanket thing around it - I guess that helps in winter)
So I think the best solution all around is for EVs to have decent 12V battery health monitoring that examines the 12V battery cell voltage and gives an indication of how much 'overhead' is remaining to reassure drivers they are no where near a problematic flattened battery on start up.