"Isn't '40' of any time unit often seen in Hebrew scripture as signifying "a long frickin' time"?"

Make it any length of time you want, though changing it from 40 and we're no longer going with literal biblical inerrency..which I'm fine with. But, the longer the time frame, then the longer the weight of the water pressing down upon the crust of the earth, the longer the water vapor would have to be present producing a cooling effect on global temperatures, etc... so it's a trade off, reduce the impact of the enormous rate of rainfall and increase the effects of ice formation in the upper atmosphere, the effects of extended massive cloudcover needed for the rain, etc...

pick your poison