Answer:
Technically speaking, over 2 liters of urine (from patients in hospitals who come in after not having urinated for a week), before pressure causes it to rupture.
Explanation:
Why I say technically is because it is rather rare for someone to simply attempt to hold it until it bursts, this is usually a medical case. If you are not urinating and you're retaining urine, it can cause metabolic abnormalities and electrolyte issues, which can lead to long term kidney failure. A generally healthy bladder can hold between 400 - 500 ml comfortably it reaches it's intended capacity.
