I researched this a month ago as I travel overseas a lot. if the large battery is fully charged it will last 3 months or more, if in doubt or if gone for a longer period, my shop
manual shows the large battery disconnect. (orange connector) just pull it out and the battery is isolated.(wear rubber insulated gloves). By the way when the orange connector is disconnected you must wait at least 10 min for the capicitors to power down before doing any repair work on the high voltage system. Small battery it would be best to use a battery tender as leaving it unhooked will cause all memory systems to be reset.

TCH-43mpg
VW TDI--48
08 LS 600 HL-20
08 LX 470-16