I don't know the code you use, but I see something in your initialization function for the ADC that an external trigger on timer is setup.
What you need to do, is understand what the initialization function does - so please read the documentation for it, or just search that function in your libraries and see exactly what it does.
The ADC conversion timing for the STM8 is very deterministic, so if you know how your ADC is setup, you also know when to expect a conversion complete interrupt.