I am sorry for being unclear again.
If you joined the site after nov you will have a daily limit of 2.66GB but if you used only 1GB on a day, the next day you can use 2.66GB+1.66GB(the remainder from the previous day).

if you bought premium for 30 days you will have a total download limit of 2.66* 30 =80GB

For account owners who joined before nov 2008 there is a DAILY limit of 10GB which if not completed in a day is not transferable to the next day.
ie if you download only 5GB a day you won't be able to download 15GB on the next day you only will be able to download 10GB maximum.