Determining exposure time for resin
Short answer: Use a calibration test.
Long answer: There are a bunch of calibration test files out there you can run. Ameralabs has a guide on how to read one of them and they have a link to download the STL at the bottom of the website. In short, the test file will print with a bunch of features that are hard for the printer to handle (thin posts, angles, small gaps, etc). You can try printing a test file like you would any other file while just guessing at the exposure time. But some printers have a setting just for this where you can run the printer once and print 8 copies of the same object each with a different exposure time. This is the best way to go if you can. either way, you will need to look at the prints when you are done and compare it to what it was supposed to look like. Pick the print that does the best in all types of features on the test.
here is a video showing how to do the test with multiple prints at once. Anycubic Photon S - Resin Exposure Test - R_E_R_F with custom timing
running these calibration tests can be useful even when you have a data sheet for your printer, it can account for other variables such a temperature. I don't usually bother but if you want to spend the time to get the best print you can it's a good idea.