Trademarks are not like patents, where you have to file within 1 yr of selling a product. You can file a trademark whenever you want after you have started using the mark/scent in commerce, even if it’s 40 yrs after you started using it.
As to the Play-Doh clone, that is a good question. The clone manufacturer may be able to argue that the trademark is invalid because Hasbro has not been using it exclusively over the years. This is a good argument if the clone manufacturer has been using it for decades before the registration issues as opposed to days.
But ultimately the clone manufacturer would have to decide between fighting a multi-million dollar fight or change the smell with a different fragrance. Considering that, if the clone manufacturer wins, it will have helped other clone manufacturers (who didn’t spend the money) to continue making their product without changes, it is more likely that the clone manufacturer would either stop using the Play Doh fragrance (giving you a plain smelling dough and a lower manufacturing cost) or switch to a different fragrance.
