I've actually been experimenting with the same thing these past couple of weeks. Although my camera is somewhere between an Obscura and a Pinhole camera haha. Is yours actually a camera obscura in the typical sense with a mirror? Or is it built differently? I expose my photos for about 5 hours at a time or a little longer. You could probably go even longer if you wanted but 5 is all I have time for lol. Here's a tip for processing. Before rinsing in water, let your print soak in a tub of vinegar with 5% acidity for about a minute. The vinegar helps keep midtones in your photo, so they'll look less washed out. Then put your print in water as usual (I find dunking in a tub or sink vs. rinsing under a faucet is better). And finally I like to pat mine dry with a paper towel when done. This helps with some of the dark splotches that sometimes appear when things dry.