For the record, tubeless sealant itself is merely liquid that is designed to prevent punctures in a tire by sealing all the small holes. However, it is not going to last forever once you apply it. But, determining how long it is going to last will depend on certain variables.
For instance, the climate conditions you ride in can play a role. Additionally, the type of tires you have and the brand of tubeless sealant will also be necessary. Overall, you should check the sealant on either a monthly or bi-monthly basis.