The answer is simple. Right when you start feeling like you need a bit of extra help.
It is a harmful myth that your first year of Junior College will be a Honeymoon year, and you only need to start getting serious about H2 Math in your second year. The truth is that you need to bring your A game from Day 1.
You see, the bulk of Pure Math will be taught in JC1, and it is critical that you spend the time to make sure your fundamental concepts are as strong as can be. After all, a tower is only as strong as its foundations.
As the adage goes, a stitch in time saves nine, and it is way easier to get it done right the first time than having to go back and relearn the basics later on.