If it has a 1ms (1 KHz) ticker, count those off for the desire period. Perhaps use a TIM to count off minutes? Perhaps an RTC with an ALARM?
Which STM8? I think they make more than one, and they often have slightly different feature mixes.
https://www.st.com/resource/en/application_note/an3133-using-the-stm8l05xxxstm8l101xxstm8l15xxxstm8l162xxstm8al31xxstm8al3lxx-realtime-clock-stmicroelectronics.pdf
