I have a few project managers that I answer to, and therefore, every now and then I'll have to work out prioritizing my work, thus, delaying the work handed to me by other project managers. The problem is they usually say something like "Have this to me by Friday" when it's unrealistic to even have it done within the next two weeks regardless of whether or not I have other projects to work on or not.
I don't want to start telling them I cannot do their projects, however, because that would ultimately lead to someone else getting the work and having me look like I don't care about the job. I just need a professional way to give my project managers a realistic outlook of their time frame.