How long does it take for a 40mg shot of depo medrol (methylprednisolone) (injected in the bottom) to work for allergic rhinitis and sinus infection??

Hours: depomedrol starts to work in about 6 hrs but the peak effects will come many hours later. Do note that it does not always work for rhinitis. As for sinus infection, it may be used along with an antibiotic but by itself the benefits will at best be temporarily without the infection eradicated. Note that this is not the customary treatment since nasal steroid works at least as well.