The 12 v battery gets charged by the vehicle's DC-DC converter from the main traction battery when the vehicle is powered on. Hence if you are not driving your vehicle regularly (or at least turning it on but not going anywhere), it isn't charging the 12 v battery. I assume it will also be charged when the main vehicle is being charged. But again if you aren't using the vehicle much then you aren't charging it frequently either.
The 300e operates in the same way as the 250h. The 12 v battery powers the alarm, keyless entry system and a few minor systems whilst powered off which slowly drains the battery. It will last over 2 weeks without problem, but you need to use the vehicle enough to put back that 2 week loss. What the 300e doesn't do is use the EV traction battery to power the standby systems and nor does the vehicle power itself up and recharge the 12 v battery if it is getting low, unlike some EVs.