This will depend on your hair length and hair thickness. A standard portion is 25g, which is about the size of a golf ball (2-3 squirts into an open hand).
Some people will use less, some will need more. The important part is ensuring that you apply your Hello Hair from roots to ends, as the ingredients in the Hello Hair Hydrating Mask benefit your scalp too. However, if you are prone to oily hair, you might find that applying only to the ends suits you better (for this you will obviously use much less).