What is the recommended exposure time for presentisized positive photoresist board?
You expose a test piece of PCB material, sliding a piece of, say, cardboard, along every 1 minute. Develop and etch it, and see which exposure time gives the best results. With high-intensity UV, the exposure time will be relatively short, and 1/2 minute intervals would be better. Over- and under-exposure will be obvious.

UV LEDs will work.