Seneca III "typical cruise" setting: 30 in. MP/ 2200 RPM / 150kts TAS / 10 US Gal/hr.
I've logged 3,000 hrs+ on seneca IIIs and found that the average burn-off was close to 80-85 Litres per hour (including taxi fuel), whether it was a 20 min flight low level or 3h30 trip with a FL100 cruise.
A fairly good and simple-to-fly workhorse.