If you said the spell for a mermaid tail how long will it take to grow?

It will never grow. Magic spells are not real and mermaids are not real. People can not change into mermaids or grow a tail. Mermaids are from stories, games, TV shows, books, and movies they do not exist in real life. If you want to be a mermaid go get a mermaid costume because that is the only way you will be one.