After drinking 1 liter of seawater with 3.5% salt your kidneys need to excrete at least 1.5 liters of urine in order to maintain normal blood sodium levels, which leaves you with 0.5 liter of negative water balance. This occurs because the kidneys have a limited ability to concentrate urine; it's explained in more detail here.
When a negative water balance results in 1% loss of body weight (0.7 kg in a 70 kg person), which could occur after drinking of about 1 liter of seawater, you can already experience dehydration symptoms, starting with thirst, dry mouth, pounding heart and dark yellow urine, followed by fatigue, dizziness, nausea, headache and eventually coma and death, that can occur at about 10-15% loss of body weight (7-10.5 kg in a 70 kg person), which could occur after drinking 14-21 liters of seawater (Nature.com).
Death occurs due to acute kidney failure resulting in the accumulation of sodium, urea and other substances in the blood.
It is not time, but the amount of seawater that causes problems. One could drink 1 liter of water in the evening, need to urinate several times during the night, and be mildly dehydrated and otherwise "fine" in the morning. But what's the benefit of this?