Fluctuation probablities?

[12AX7]

Hi Everyone,
I'm sure this has been done elsewhere. But here goes.

Suppose a player is breakever, how often should he expect a 10 BB downswing? 20BB? etc.

Suppose the player is the hypothetical "expert" at 1BB/hr (or what ever the equivalent is for 100 hands per hour, the online measure).

What I'm trying to figure out, is how much of a downswing is typical for a session of say 4 hours, 8 hours etc. so I can gauge how much is just the inherent variablity of the game vrs. how much of it is the "I suck a HE" factor. LOL!