Absolutely love the precision! Very wide SD - I wonder if you've got a bit of a bi-modal distribution based on whether you're just topping up or going further and need the full charge.I'm one of those boring people who has it all in a spreadsheet, so I can tell you that the arithmetic mean is 15.07, with a standard deviation of 10.93 the median is 13.13 and the mode is 5, cos it put 5kWh in twice ;-)
Shorter answer would be - same as Tooks. As much as I need, plus a bit more for good measure! More if it's free/ cheap/ fast/ somewhere pleasant, otherwise as little as possible.