I think its going to differ between species groups and habitats within the classification of detritivore. The fact that plants are not significantly consumed by terrestrial herbivore suggests that there is lots of dead and decaying material about for terrestrial detritivores. Moreover, the plants keep producing more, so its not like it runs out (at least not unless there is a major perturbation). Of course there are issues like C:N ratios but most (all?) detritivores consume microbes and fungi as part of their diet as well. The situation may be different for species that focus on other resources (dung beetles), and certainly in aquatic systems there is often no significant standing stock of primary producers, so there could be limitations there (e.g., studies on stream leaf shredders often point this way), although in open pelagic systems the “standing stock” is really secondary production, and that is what detritivores would focus on.
I think its going to differ between species groups and habitats within the classification of detritivore. The fact that plants are not significantly consumed by terrestrial herbivore suggests that there is lots of dead and decaying material about for terrestrial detritivores. Moreover, the plants keep producing more, so its not like it runs out (at least not unless there is a major perturbation). Of course there are issues like C:N ratios but most (all?) detritivores consume microbes and fungi as part of their diet as well. The situation may be different for species that focus on other resources (dung beetles), and certainly in aquatic systems there is often no significant standing stock of primary producers, so there could be limitations there (e.g., studies on stream leaf shredders often point this way), although in open pelagic systems the “standing stock” is really secondary production, and that is what detritivores would focus on.