Variance Problem

[JOHNY CA$H]

Hey guys, this type of math is a bit foreign to me, so I was hoping you could help me out. I don't need an exact figure, just a rough estimation. How often should this scenario occur in a year? Ten years? Over a 40 yr career? What is the rough probability of it occurring?

Assume a full time professional online NL (200-400) player. Plays 25-30 hrs a week, 5 tables, and averages 5BB per hour per table (25BB per hr), or 1/4 stack per hour. How often should he:

1) Experience a 20 Buy-in downswing?

and 2) Experience two 20 buy in downswings within the span of 4 months? (Is this something he'll see every few years? Every ten? Once in a lifetime?)

Thanks in advance, I'd love to hear anyone's thoughts.