Potty training tips please

I have a 3 years old boy he is my first born and I started potty training. So far everything is fine. I want to know how long does it take for a toddler to start peeing and pooping in the toilet? My son understands that when you set on the toilet it means you are doing pee pee or poo poo. Also, is it normal their peeing and pooping schedule changes during trainin? And is it normal for them to only have accidents the first few days and not peeing in the toilet?