Hi Icing,
My experience here is that if icing is encountered during the descent, you use the engine anti-ice systems as advertised, with the minimum N1 RPM (depending on the engine type).
With the de-ice systems, either one of the pilots, (with the FEs promting), looks out and checks the wings LEs (retracted), for the build-up of ice, and uses the bleed air to dislodge that ice, then turns off the bleed, and wait for more build-up, that way, the ice only builds up on the LE, not the upper/lower surfaces.
I agree, holding in icing conditions, no LEs, and use de-icing system as required, each 'burst' of LE bleed air lasts only for about 20-25 seconds to do the job.
Anticipation is the secret here, when you are manourvering to start the approach, just prior to selecting flap one, give the LEs a de-ice cycle, and that has to last until touch down.
Cheers