The longest protection period 15 years means that the protection period is 15 years from the date of design completion. If you apply for a patent in 10 years after completion, the protection period is only 5 years. If you applied for a patent on 16, then your patent right has been lost and become the only one.
If someone uses your patent during the period from design completion to patent application, you have no right to take it back after patent application, but you can ask them to stop using it.