Best answer by woz
In my opinion 2 (or 3) days after you get the email reminder which should arrive 3 days before the end of your billing cycle or 5 days before the date on your statement. You may get away with the extra day but then again you may not...
So for example
from your statement "Your energy statement for the period 15th July to 14th August" email would have arrived on 12th July, add 2 days submit on 14th or 15th I like to play it safe.
It really doesn't matter all that much, the days after you submit to the billing date will be estimated, usually pretty closely, but obviously the nearer you are to the billing date (in the example above 15th) the less room for error there is.
Also you can submit readings once every 24 hrs if you wish, all the readings you submit should appear on the statement
What's the best date to give a meter reading to get an accurate bill.