here's my old 10/20 graph:
Note: I sucked for the first 30k hands so I don't really count those as a downswing because I wasn't playing good poker.
Depends on how you think of a downswing - you can find a couple different areas that could be considered ~25k hands of breakeven. I don't think there are many spots that exceed 10k hands of consistant losing though.
Here's my more recent 20/40-100/200 graph.
Again, regions that are ~15-20k hands could be considered breakeven poker. Not many constant losing runs >10k though.
edit: to answer your questions - combined both my graphs, I see about 6-8 10k downswings, with prolonged areas of breakeven. Yes, these are very normal, I would say within 40k hands you should expect a 10k stretch of breakeven to losing results. With a reduced variance style of play, this could be reduced, but overall bb/100 would drop as well.
-DrG