Hey, it really depends on many things:
Do you feel that you might "forget" these UCAT skills over time? I know for me, it's been a few months since I did my UCAT and I can strongly say if I did it again right now, I would not have gotten the high score I got. If so, maybe consider starting proper practice 2 months before.
Are you paying for resources? Things like Medentry, Medify are paid. Are you able to pay for these resources for a long period? Official question banks exist too, but I highly recommend you save these until the weeks before your test day.
Finally, do you have the time to actually do UCAT practice consistently at the current moment? If so, go for it, but you may have other priorities too look at for now (e.g. work experience, school subjects etc.)
Overall the aim is to do things consistently, and no matter what, it is a good idea to familiarise yourself with how the UCAT works ASAP so it isn't as daunting when you get to practicing.
Hope this helps, this is my opinion, might not work for you!