When washing your hands for Netilat Yadayim, how much water do you need for Netilat Yadayim?
Shulchan Aruch, Orach Chayim 4:6, says there's no minimum amount of water needed for washing before shacharis (or, by implication, before mincha or maariv or on arising).
SA, OC 160:13 ff., says you must have a r'viis of water to wash hands before a meal. The conversion rate of r'viiyos to milliliters (or fluid ounces) is a matter of dispute, but 90 milliliters or 3 fluid ounces is probably sufficient if you have no reason (like your community's practice) to demand more. Please consult your rabbi for such reasons and, in general, for practical guidance, rather than relying on what you read on this site.
Of course, the water must also be sufficient in amount to wash the surface it's washing.
 Safek d'rabanan l'kula.