I don't know about doing the Duchess or doing touch and goes, but here's what the Seminole information manual says:

Perf cruise
55% - 17.4 gph
65% - 20.4 gph
75% - 23.3 gph

Econ cruise
55% - 14.0 gph
65% - 16.6 gph
75% - 19.2 gph

I usually flew it at econ, it's only like 2 knots slower.