We bought the Intensive Practice series[0], and paced study time at a couple of pages a day (which takes about 20-30 mins), semi-supervised (i.e. kids mostly work on their own, but if they don't understand something, we help clarify). It does take some hand holding at the beginning though.
It isn't related to school curriculum (our kids go to US public school). It's supplemental in the sense that they get to practice more math exercises than a kid that only relies on school/common core curriculum.
[0] https://shop.singaporemath.com/index.php/product-category/su...