I got a referral after TTC for 6 months, was a 2 month wait to get in, first two months have been monitoring my cycle so it doesn't hurt to get a referral early given you can lose another few months just getting in and started with everything. If something feels wrong then there is no point waiting. My cycles were irregular and I know something is not right so no point waiting 12 months+.
Unfortunately after my 2 monitoring cycles and lots of ultrasounds and tests they've said its unexplained and I can keep trying (given it's not yet 12 months) or go to IUI or IVF for best results. Given we're still less than 12 months we'll keep trying until the 12 month mark and then consider IVF. I'm also getting a second opinion in the mean time.