Did it last weekend with 0 prep. Tbh I don't think I did that well bc the reading section really confuses me, like I swear either always multiple options seem legitimate or none of them do...
The Math subject tests are A LOT easier than AS Maths meaning that you can probably go in and get an 800 without much prep, other than the fact that there are 1 or 2 topics not on the AS syllabus but you don't even notice. Nothing I hadn't done or couldn't understand without having learnt it came up on my Math 2. The scaling is also in your favour you can get as low as 43/50 and still get 800.
English subject test is similar to the reading portion of the SAT, so you don't have to learn anything beforehand it's just about analysing what they give you. But I'm terrible at that but if you're not do that.
The sciences (at least chemistry) look very difficult there are like 80 questions in one hour, while maths has like 50 questions, and the questions extend further than the AS syllabus. Also the scaling is not in your favour, to get 800 you need 100% and dropping a few marks knocks quite a bit of your score. I think it is different for Physics and Biology however.