Does invitation letter has an expiration date from the date we issued to the date they apply visa?

Hope this mail finds you well.
I'm Alice from Etman Electric, China. Our company have sister companies in Finland, In this September we will invite them to China to celebrate our 10th anniversary. Here we find a question that make us confused and would like to bother you to give some guidance.can you give some guidance?
About the invitation letter we issued for them Finland to apply entry permit, does it has an expiration date from the date we issued to the date they apply entry permit? For instance, if we issue the invitation letter on this March, but they use it for entry permit application on August or even September, will the invitation letter be expired at that time?