The more you keep toffee on fire the harder will be when cooled out instead of being sticky.
To know when it has the texture you want keep a glass of cold water near you and pour a drop of toffee in the water when you think it might be ready: if the drop melts it’s too early, if it’s sticky but too soft wait few more minutes, if it’s gummy sticky it’s ok, if it’s even harder you’ll have hard candies ( they will last longer).