Maybe 5-6 hours if you want to power it, or up to 10 hours if you want to enjoy the surrounding beauty. A lot of people (ourselves included) camp overnight to make the most of it and explore a bit further up the river.
Falling rocks are the biggest danger and have cost a few lives here over the years. As with a lot of Taiwan's mountain streams, flash-floods are always a risk, so be weary of the weather.
Helmet is a must. Then of course tracing shoes. A dry bag if you want to carry stuff all the way to the grotto, but you can get to within the last 20-30 min of the grotto without needing to swim (but you might slip and fall in lol). If you are not a confident swimmer, then a life vest would be good. You will have to swim across a couple of deep pools to get to the Grotto.
Beautiful trace with deep pools and stunning canyons. The Golden Grotto itself is a 20min walk up a side stream after a 3-4 hour walk up the Sanzhan River. Right before the grotto there are a couple of deep pools to swim across and a couple of waterfalls to climb (but they seem to always have ropes installed). It's quite a popular trace with several parties going up per day on the weekends. It was not difficult, but requires a bit of fitness. You can even get there by public transport. I took the train there and back and was able to walk from Jingmei station to the trace entry point without much trouble. There are a couple of little shops in the village, but food is scarce. Make sure you supply before you go.