Despite Tesla recommending plugging their cars in when parking for extended periods I had to leave my Model 3 unplugged when I went to Sweden over the winter holidays.
Before heading to the airport I charged the car to 85% to make sure it wouldn't run flat. During my trip I tracked the so called “phantom drain” from the battery by taking a reading of the charge level every evening using the Tesla app.
After 42 days the charge level had only dropped by 28 points, to 57%. That's on average about ⅔ of a percentage point, or ½ kWh, lost per day. Based on this I wouldn't even worry too much about leaving the car unplugged for up to three months as long as I've charged it up beforehand. But plugging in while parked is of course always preferable as it lets the car run its battery management system.
An interesting data point to note is that one day after starting the experiment the charge level had increased to 86%. I suspect this was due to the battery recalibrating after the charging session.
Car specification: Long Range Rear Wheel Drive Tesla Model 3, firmware version 2018.42.4.
Environment: Unheated indoor garage in Seattle during December and January, temperature range 0 – 15°C (32 – 59°F).