I am not aware of any publications addressing this question, I think it depends on how specific the behaviour is that is needed to collect the resource on one plant species compared to that for other plant species that are available. If these behaviours are very different, than the bee is likely to be more constant, regardless of whether it is collecting pollen or nectar. Bees can also collect pollen passively (not by buzzing or any other active behaviour) during nectar foraging. Furthermore, they will always need some nectar (fuel), so a bee that collects pollen on a nectarless species could also switch from time to time to get nectar to keep going.

I think it depends on the floral resource, for instance collecting pollen from date palm often constance and take short time, as well as gathering nectar from banana.

I have never observed differences in the floral constancy between nectar and pollen collection. I could see that this constancy fails when the honey bee has only available flowering plants usually unattractive and also little represented in the environment.