The monkeys come out both early in the morning and late in the afternoon (4-5pm), but if you come in the afternoon bring bug spray.
Try to go on a weekday - the crowds on weekends freak them out and they don’t always show themselves.
Also, there is another way that doesn’t involve climbing stairs for a half hour. I love hiking, but climbing stairs is not hiking.
Go to Jiantan and exit towards the night market, heading left as you leave the station, to the row of bus stops. Take Red 5 or any other bus that says “Yangmingshan” (I think there is at least one other, maybe two) and get off at Wenhua Daxue (Culture University). Turn left away from the main campus area to a little settlement with a few shops and a pretty Chinese-style church. Turn right at the church - which looks like a temple - down the hill and signs will lead you to a set of stairs.
Go down those and when you’ve hit flat ground, you’re in the monkey area. The best monkeys can be seen at the other end of their territory, but it’s a lovely walk.