It has to be very high, because our uncertainty about the future is absolutely enormous. I'd put the discount rate even higher than the monetary discount rate, which with fairly standard numbers is already effectively 0 in 20 years.
It takes a ridiculous discount rate to effectively become 0 in 20 years. 5%/year * 20 years = (1-.05)^20 = 35.8%. 10%/year * 20 years = (1-.1)^20 = 12.6%.
Still, 100 years is often considered a reasonable limit on such things as 5% * 100 years = 0.6%.
