It needs lots of exp. Still,keep in mind that monsters give more and more exp. At level 85 you get millions of exp,at level 105 areas like sea of spores billions. And you need ~900 billion to go from 98 to 99. That's like few hundred mobs in sea of spores. And I'm pretty sure that going higher mobs might actually give you like 10-100billion exp or even 1trillion\kill. Still...even with such exp gains getting ~400.000.000.000.000.000 you'll need to kill at least 400k monsters to go from lv 119 to 120. But I really doubt it takes 20 years.