Usually about 3-6 weeks but it can be longer if she cant find an appropriate lay site. Get a couple of clear tuperware containers or something similar, cut an access hole in the top of them and 3/4 fill them with damp spagnum moss. You may have to paint the top and sides black so that she feels more secure in ther but dont paint the bottom that way you can easily check for eggs because they usually lay them on the bottom. I incubate my O.castelnaui at 28.5c (should be the same for marmorata)
Does your femaly look like this?