You should charge "per month/year", if you have any intention of supporting your product and your customers long term. Sell the library for X EUR, and then cha

You should charge "per month/year", if you have any intention of supporting your product and your customers long term. Sell the library for X EUR, and then charge Y Eur/year for ongoing support and maintenance, where Y is a fraction of X. If the user does not want the support and maintenance, so be it, they can use the software indefinitely. In my experience, most serious users have no problem paying for long term support. Good luck.