Your hair extensions can be washed less then your natural hair. I wash mine every two weeks or if i have styled them heavily for a special event. If you have oily hair make sure to spray the roots of the extensions with hair powder lightly every couple of days to make sure the glue stays intact. Maintaining your hair extensions properly will allow them to last years and years!
So general rule of thumb: no oil, no oil, no oil!
Remove all your hairies and tie them with a rubber band at the top:
Using cool water, not hot not freezing, fill your bathroom sink with enough water to fully submerged the hair:
Let all the shampoo water drain out and rinse the sink out well and turn it on to wash the hair making sure the water is again cool in temperature
lift hair up from the top and squeeze out access water. (You do not condition extensions, conditioners have oil in them. You can if you want find a silicon based anti tangle spray but i don't use it. I once overly conditioned some extensions and from then on they constantly became tangled and knotted, it was terrible so don't do it!)
squeezed out hair.
now using a towel place the extensions in the towel and gently pat them dry