james i usually use around 300-350L a day.i only work a max of 6 hours "on the glass" per average working day.i have 2 barrels which take me up to 400L if needed.
i might just put my existing tank in my new van and 2 barrels so ive got a 400L to start the day with.
to be honest i lose interest after 3pm-4pm and start heading off home to fill up for the next days work,print invoices,text customers etc.
for add ons i usually hook up to custys outside taps with 20m of extension hose.
regards