When buying plaster, purchase only what you require, as it has a very poor shelf life. Most builder providers and DIY outlets have the handy pack size plaster suitable for small patching jobs. Some builder providers often sell plaster by the stone over even a half back always enquire as not to purchase more that you require.
A rough guide is as follows
The 10kg bag should cover about 1.5 sq m at a thickness of 10mm.
The 10 kg bag should cover about 5 sq m at 3mm thick.