You are not permitted to store personal data too long. When it´s no longer needed for the purpose for which it was collected, it should be deleted. If a person ceases to be your customer, you should delete  the data relating to that person. Normally you can save data relating to a previous customer for one year in order to promote yourself and get that person to become your customer again.

In order to take care of warranties, necessary data can be stored until the warranty has expired.

There might be other requirements for how long you should store data, for example for your bookkeeping.