There’s no definite number for this because a lot depends on blood circulation, flexibility, and personal endurance. The way you’re secured is also important. If you’re tied too tightly, you will go numb very quickly, which won’t paint an accurate picture of your tolerance. So, you must be secured properly.
Generally, some people go up to 15-20 minutes before they start going numb, and others can do hours before numbing. So, it’s up to the abovementioned factors how long it takes for you to go numb. Don’t hesitate to call your safe word once you’re in that situation.