period - When should an adjective be followed by a comma?

This happened whenever she was left alone in someone else's home.
She'd feel as if she had been put inside a huge, abandoned turtle
shell.

I don't know why, but I just put a comma in the example above (I think I learned this in some writing course.)

I searched for a similar sentences on Google and found this:

Living in a huge abandoned turtle shell called Big Green

So I'm a bit confused, when should an adjective be followed by a comma?

Answer

This concerns what R L Trask in his ‘Guide to Punctuation’ calls a listing comma. You might find the entire passage on commas helpful, but the summary of his advice on listing commas, which applies to lists of adjectives as well as nouns, is:

Use a listing comma in a list wherever you could conceivably use the
word and (or or) instead. Do not use a listing comma anywhere else.

In your example, it would possible to write ‘a huge and abandoned turtle shell’, so that means you can write instead ‘a huge, abandoned turtle shell’.