The amounts of milk women are able to express are very individual, and will vary for each woman, depending on many physical and environmental factors. If you are having breastfeeding difficulties, and need to express for comfort, this may only require you to express a few drops, to a few millilitres (you can probably do this in the shower or bath, or let the milk fall onto a clean nappy or hand towel).
When expressing larger amounts, the amount you express will depend on:
Your individual body
When your baby last fed, or you last expressed
How old your baby is
How often you are expressing
Your 'let-down'
The timing of day
Other factors
Your individual body. Some women find it easy to express 40, 80, 100 mls (or more) at one session. Other women find that 20 to 30 mls is their limit. A few women discover that even though they have plenty of breast milk for their baby to thrive and grow on, trying to express milk becomes a difficult and frustrating exercise. If you are unable to express much (or any) milk, first check with your caregiver or early childhood nurse, to make sure you are expressing correctly. It can sometimes take a while to master. It may be that you can express using a breast pump, but find it difficult to hand express (or visa versa).
A few women find they cannot express any milk, no matter what they try. This is very normal, but you may have to accept that expressing milk may not really be a viable option for you. On the whole, babies are much more efficient at removing milk from the breast than we are at expressing it, meaning that your baby is getting what they need, but you are unable to express much extra.
When your baby last fed, or you last expressed. The longer it has been since you breastfed or expressed from a breast, (in terms of hours, not days) then more milk will generally be present to express.