Is 8 weeks enough time for fertility test?

I've just had a vasectomy a couple weeks ago and the NHS has sent me a letter confirming my fertility (or I guess infertility) appointment set for about 6 weeks from now - so just over 8 weeks since the procedure.

I've been reading that it usually takes at least 90 days and 30 ejaculations before you're fully sterile. I'm debating whether to reschedule or just try and hit the 30 mark before then and hope for the best.

Has anyone - especially in the UK - had the test sooner and what were the results?