Gambler's Fallacy
Seriously though, if you're "expecting" a downswing in the near future, wouldn't it be better to just not bet at all? You've got to move past this line of reasoning.
Personally, I modify my unit size every single day, according to my bankroll. While it may not make a whole lot of difference, I like to think that every bit of precision helps. But this may seem unreasonable to some, so just do whatever works, whether it's a certain timeframe or a certain target bankroll.