My attempts last year softened within a few hours. This was because I did not cook the toffee to the hard crack stage - 150C/ 300F. I hope my tips will help you achieve success, but if something goes wrong and you end up with soft toffee, don't despair. Try storing it in the freezer until ready to eat. The freezer firms it up nicely and, once it's in your mouth, you won't care if it's not rock hard. It will still taste wonderfully rich, buttery, sweet and... treacly.