IBUs are calculated by the formula
IBUs= Hop Wt in oz x % Alpha Acids in hop var. used x Utility factor (based on boil time and some other factors will range from 30% to <5% typically) x 7489 (a factor that converts metric to US units)
The product of these 4 numbers is divided by volumn of wort.
There may exist some upper limit but due to soluability of isomerized alpha acids in wort but it exceeds 100 IBUs.
Try to sample some IIPA examples look for the following Imperial IPAs:
Russian River Pliny the Elder, Three Floyd's Dreadnaught, Avery Majaraja, Bell's Hop Slam, Stone Ruination IPA, Great Divide Hercules Double IPA, Surly Furious, Rogue I2PA, Hoylan's Hopsickle, Boulevard's Doublewide IPA
Although these beers are a showcase for hops they still maintain drinkability.
We have made a clone of a couple of these with IBUs calulated at 150+. As for how we humans percieve these high levels you can decide for yourself. Most experienced beer judges consider 100-120 their upper limit with 5 IBUs as the smallest variation that can be detected (they could determine 75 IBU beer was hoppier than an 70 IBU sample of the same beer). There are some tricks to making hop bitterness more or less perceptable. Adding sulfate ions (gypsum) to the brewing liquor is common. I know a brewer in Colorado who swears that he can brew a 30 IBU beer that will seem more bitter than a 45 IBU beer.