What was the lift-to-drag ratio for WWI biplanes?
AE Berriman published his respected survey of Aviation in 1913. At that time drag was known as resistance, with other terms such as head resistance or drift also used. He discusses the lift:resistance ratio only with respect to aerofoil design.
It is unlikely that the l/d of the whole aircraft was ever considered quantitavely as a useful number. One might gain some idea by looking at Farnborough wind tunnel results for propeller thrust at given airspeed and RPM, and then comparing those with aircraft fitted with that propeller, but that would have been of little value in a hectic war and would not have been done. Of more use was the relative speeds of different types when given the same engine and propeller, or of a given machine when fitted with a new propeller.
Pilots would have had some idea of how far their machines would glide when the engine failed, as happened often enough, but only hazy notions of actual speeds and sink rates. They would also have struggled with the fact that the gliding speeds and angles for minimum sink rate and maximum distance were different.
With hindsight we can make guesses. Glide slopes remained well below 1:10 until the German soaring gliders of the 1920s. Airspeeds were slow, around 100 mph, so thrust and drag were not as high as might be expected today. While engines had relatively poor power-to-weight, airframes were so light they would sometimes break up under the extreme manoeuvres of combat. A steep gliding angle of say 1:4 would have required an unrealistically powerful engine to generate thrust (nominally) equal to a quarter of the gross weight, so wings were relatively large and lightly-loaded to give better lift at lower speeds.
So I'd hazard that around 1:5 to 1:8 would have been the norm. If anybody knows of a better-researched analysis, I would be delighted to be proved wrong.