Hi Karin, fully understand your anxiety.
There isn't any specific time. However, coming to 2, should be ok for you to start to try to train him.
Probably about an interval of 20 -30 minutes.
Most importantly, you need to communicate to him what are you doing.. and why are you doing this. Next, dont't pass your anxiety to him.
That's was my past experience with my 1st, so anxious to 'achieve' something.. and you know what... my girl responded...' I tried my best Mummy, nothing comes out.'