How long does it take for real ugg boots to mold to your feet?

I have the �150 cardi uggs. I have just bought them from OFFICE and at the moment my heel is slipping uo and down as I walk. The lady in the shop said they're not too big and this always happens and it takes time for the boot to mold to your foot. But HOW long does it take? Cos if the boots are in fact too big I will have to get them changed