Trying for the second baby advice!!!

Le

Ok so it was 2016 (October) I had my none hormonal coil taken out. Me and my partner have been trying ever since, 18 months. Not conceived yet! My question is, how long do I leave it before going to see a doctor? I fell pregnant with my first after 3 months. I'm posting from the United Kingdom 🇬🇧. Tia