How long to die if I withdraw blood with a syringe over and over again?

[Ambivalent1]

?

 

[Takamagahara]

You will most likely fall unconscious from hypoxia before it becomes lethal. You'll be weak and anemic for a few days but you would survive as your body would gradually find a way to deliver oxygen to your brain.

This is why you would need to slit a major blood vessel or arteries to ensure continuous blood flow after you're unconscious to induce oxygen deprivation in your brain.