It depends on what kind of QFT calculations you want to see... a good QFT book ought to have quite a few.
I don't know of "the longest" calculations, but I suppose you could make them as long as you want, for example by pertubatively expanding some arbitrary QFT to some arbitrary order and doing the integrals. For example, Standard Model quantities have been numerically calculated up to some large order in the couplings. However, those are kind of "trivial" cases: although the calculations are not pretty and become longer and longer, the method is straightforward and well-studied.