In clear water, what do you guys think is the maximum depth water that is pratical to fish top water plugs?  In other words, how far up will a bass come to take a bait on top of the water?

On Lake Erie I've seen fish come up out of 20' to hit a topwater.  Can't answer for Kentucky Lake....cuz I haven't ever seen the bottom :

Remember just because you catch a fish over 20 or even 30ft of water does not mean that the fish was that deep. They may be suspended or way up in the water column to begin with. With that being said fish will travel a long way. The bigger the top water the more drawing power.

I've caught fish in 50' deep water on topwater baits, as Gobig mentioned those fish are usually suspended and did not rise through the entire water column. I regularly take them in 20' or less.