How long do most people prepare for STEP

Kinda depends how you're finding it, but usually the advice is to start looking the summer before you're due to sit it because it's a very different style to get used to, and of course challenging.

I need to get an S1 and I'm beginning my preparation now. I've done the 25 modules but that's it. How far behind most people am I. I didn't realise just how early I shouldve started prepping.

A whole year in advance is much earlier than it seems most people start preparing; I would say most people begin properly preparing after they get an offer. They may have a look earlier, as you did say in fairness, but the summer before still seems quite early.

Don't focus on how far behind you are - you'll need to spend a set time each week on prep as 50% offers are missed on that.
Also do ot look up solutions too quickly either - struggle a bit!

I believe the OP is on a "gap year" (so a levels last year) so they should be able to enjoy step questions 24/7 now?
I guess the usual thing is to try and work back from the deadline (say end of may) and say theyve 6 weeks for step 2, 6 weeks for step 3 and a couple of weeks to try a few step 1 (for warm up). Then think about maybe covering a dozen years of step 2&3 in that time and some database practice on specific stuff.
So probably enough time if they use it effectively.

Yh probably should've mentioned I'm on a gap year lol. However I'm also working and planning to travel so I don't have all the time in the world (although ofc I'm still not having to juggle 4 A-Levels).

So you have about 4 months time left. How do you plan to use it and how much time are you devoting to step?
The 24/7 was tongue in cheek, but starting now for a S/1 level will require a fair bit of devoted time.

I'm thinking 3 hours a day. Probably more in May, and I'm also trying to do more than 3 hours now (although I'm still recapping all the content cos I've forgotten a lot). However, if I go travelling (no plans set in stone, likely in April) then I'll probably only able to fit around 3 hrs.

Not unreasonable, but a back of the envelope (at the start) is 3 hrs a day is maybe 3 questions, so assuming youre planning on practicing on the whole papers (12 questions) its somewhere between 1 and 2 papers a week. So for step 2 and 3, youll probably get through a dozen or so years worth during that time, assuming you keep at it.
Have you tried a step 2 and a step 3 paper to see where youre at now?

I'm half way through the step 2 support modules. I'm both going through the content and doing the modules and then hopefully can get to the end of the step 3 modules In like a week. Then I'll do my first paper I think. (Assuming I can actually stick to it) are you thinking 3 hrs is enough or too little?