These spheres were placed into a glass of water for 5 to 7-days.
These spheres were placed into a glass of water for 5 to 7 days.
The spheres were baked for 5-days.
The spheres were baked for 5 days.
Which is the correct way to write days? Thank you.
Personally, I'd not hyphenate at all in this case. It's not a recognised phrase that's come to represent a single word, such as 'a seven-year-old'; it's a number of things, just as 'two pipes' or 'three *****'. Also, with lower numbers (it's a moot point as to what number you do this to - 20 or 100 are suggested) you should write the number in full, not just use the digit.
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Personally, I'd not hyphenate at all in this case. It's not a recognised phrase that's come to represent a single word, such as 'a seven-year-old'; it's a number of things, just as 'two pipes' or 'three *****'.
Also, with lower numbers (it's a moot point as to what number you do this to - 20 or 100 are suggested) you should write the number in full, not just use the digit.