I've just been looking at some Energizer and Duracell data sheets and they seem to confirm what I've found in practice - that modern alkaline batteries really are very good.
Taking a radio with xx96 series valves the total LT current would typically be 150ma. Equating to a 10 ohm load.
Looking at the manufacturer's data sheets for the same load a D size (LR20) will give 80 hours service when used 4 hours per day to a 1.2 volt end point. Somehow I can't imagine an AD35 with it's two Zinc Chloride cells would do better than this.
The smaller C size (LR14) is also good for about 32 hours on the same basis.
With the older xx91 series valves and a total current of 250ma the D size (LR20) is still good for over 30 hours.
I know I'm pushing my luck, but the single C size (LR14) in my Marconi P20B is lasting well in spite of the 250ma load, though I have to admit I don't use the set for more than 30 mins at a time.