Another data point can be gleaned from:
https://www.americanradiohistory.com...ion-Manual.pdf
The first chapter of the manual describes a personal superhet using the D90 series of valves. The 7.5V, 50mA filament supply for the four valves connected in series is provided by a Vidor L5059 battery. The HT battery is said to be the limiting factor in overall battery life, so I guess we can assume that the LT battery was lasting for a reasonable amount of time.
best regards ... Stef