You can try Kaggle notebooks and Google Colab notebooks but they don't persist for that long. They typically shut down after 6 hours. You'll have to periodically save your best model/hyperparameters but that might be a viable free option.

Google Colab also has a paid option where you can upgrade the RAM, GPU etc.. to meet your needs.

But I am curious as to why it's taking 21 hours. Have you checked in your course forums/discussions for the expected time ?

https://www.kaggle.com/

https://colab.research.google.com/