“How many hours can you be under anesthesia?”

There really is no time limit

Modern monitoring systems make anesthesia very safe ... even for long periods of time... most surgeries only last a few hours but occasionally there are procedures which last much longer. Liver transplant surgery, for example, may last over 24 hours. Also, sometimes patients are sedated or put into ‘induced comas’ for extended periods of time. This may be for hours or even days. Ideally, we assume that the less time under anesthesia the better, but long procedures can be done safely without apparent harm to the patient

There is really no limit to how long people can be under anesthesia.  In the ICU, people are heavily sedated for days.  In the OR, some complex procedures take in excess of 24 hours

I don't know of any maximum time limit to general anesthesia.