You should definitely hire someone ASAP. For future brides that will read this, I would hire someone about a year out. Trials are really up to your schedule. If you're going to have a shower, bachelorette, or some other special event, I would suggest doing your trial that day so that it doesn't go to waste.
Also, I agree with Caytlyn - if you have engagement pictures or a bachelorette party or a bridal shower or some other event (even a girls night out), schedule the trial for that day! You would be able to see how the hairstyle holds up throughout the day, so you know what to expect for your wedding day.
I hired my HMUA about 6-7 months out. I did a trial before I booked her to make sure I liked her work. I would book ASAP. With many 2020 brides rescheduling for 2021, vendors are booking up very quick!
I would hire someone now. I hired mine about 6 months out and the first two I contacted were already booked! I would see if your trial matches up with engagement photos or party or even just date night so you can get more out of your trial. I’m doing my trial about 2 months before the wedding.