Do you give your parents an allowance? If so, how much do you give them monthly?

I am giving 20% of my take home to my mum only and my wife is contributing $400 even though i told her that there is no need for that. It actually depends on your parent mindsrt on do they want you to give allowance or not. Some just want a sense of security that their child are filial and some will just save the money up for the kids.
So if u know how your parents and since they told u that u don't have to give, u could occasionally offer to pay for groceries or pack Ang bao for them.

When I was a fresh grad, I gave about 20% of my pay before CPF deduction.

Your parents would probably prefer you to make an effort to spend time with them, by buying them their favourite food, bringing them out (taking current situation into account) or even just asking them if they need any errands run and helping them to do it.

Perhaps it is the thought of doing so willingly that counts! Money is just only a means... in fact probably buying your parents their favourite food during special occasions like new year or birthdays during your visits or bringing them overseas for an experience they always had wanted might probably be appreciated by them too man! Not to mention topping up their CPF MA or simply bringing the grandchildren back for them to have fun with after marriage with your partner can also bring joy in their golden years :p