how long can my frightened indoor/outdoor cat hide under house before she comes out?

Do what @PR suggested but open a can of tuna fish and sit it just inside the door with the door open.  She will smell it. 

 When she is hungry enough, she will come inside where she knows it is safe. 

Try going out at night and calling her from under the house. Cats are more bold at night. Be sure you have not fed her on that day, so she is more likely to come out. 

Besides that, if you are trying to trap her, cover any access the other animals may have to the underside of the house before setting any traps. That would be especially upsetting to the cat, if racoons are coming in there, as well.

Are you SURE she is still under there? Can you see her with a flashlight? The problem is that if she was attacked, there may be injuries that could be getting infected. You need to get her out of there, right away.

They are sensitive to odor and noise of other cat in the neighborhood. Try scaring the Tomcat away. Play some music to suppress the outside noise. They can quit eating upto 12-14 hours before they calm down.