It's more usual to put the apostrophe after days, in official documents, etc., e.g.
1. ...ten days' notice...
= "notice of ten days [in duration]".
Some might argue that we can treat "ten days" as an attributive noun phrase, and omit the apostrophe:
2. ...ten days notice...
But when we use phrases of time attributively, we're more likely to say e.g.
3. ...a ten-year sentence...
4. ...a ten years sentence...
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