Basically John will die at a given moment, which is...
...before his original lifespan would have ended (Because of How to Use: LVII)
In the Death Note, you cannot set the death date longer than the victim's original life span. Even if the victim's death is set in the Death Note beyond his/her original life span, the victim will die before the set time.
...within 23 days of writing, because a specific time of death was written (Because of How to Use: XXVII, rule number 2)
If you write, die of disease for the cause of death, but only write a specific time of death without the actual name of disease, the human will die from an adequate disease. But the Death Note can only operate within 23 days (in the human calendar). This is called the 23 day rule.
In our case that would be dying peacefully instead of disease, because the actual way John is supposed to die is not specified. He could die in a coma, could die after being drugged or by sexual exhaustion as mentioned in kaine's answer.
- ...after 6 minutes and 40 seconds if none of those 23 days were in the year 2315 or if it wasn't possible for John to die peacefully within those 23 days due to war for example (Because of How to Use: XI, rule number 3)
As you see above, the time and conditions of death can be changed, but once the victim's name has been written, the individual's death can never be avoided.
Therefore, if you write John zeurtö dies peacefully in the year 2315 one of the following will happen.
- 2315 is after his original lifespan; John will die as he normally would have.
- 2315 is after 23 days of writing; John will die of a heart attack after 6 minutes and 40 seconds.
- 2315 is before 23 days of writing, but it is impossible for John to die peacefully; He will die of a heart attack after 6 minutes and 40 seconds.
- 2315 is before 23 days of writing and it is possible to die peacefully; He will die peacefully within those days on the most probable date.
Now if you would have written "John dies peacefully of sexual exhaustion", without mentioning a specific time, then he might have died later than those 23 days, if it was still within his original lifespan (Because of How to Use: XXVIII, rule number 1)
If you write die of disease like before with a specific disease's name, but without a specific time, if it takes more than 24 days for the human to die the 23 day rule will not take effect and the human will die at an adequate time depending on the disease.