Personally, I don't repitch anything that is stressed in any way. I don't underpitch on principle, but if it's in a higher gravity batch, then I won't repitch it either.
The longest I've gone is 3 generations, using 10 gallons of 1.050 beer for each successive generation, with new wort ready as soon as the last batch was done fermenting. But I'm more anal than your normal homebrewer.
For a question like that, I'd have to try and read the Yeast book again.