All the image licenses on Vecteezy are available for use perpetually. That means you can use the images forever in a single project.
If you would like to use that same image again in a new project, you must download it again to re-license it for your new project.
You may not download and stockpile images for future use. You may save them on your computer for a duration of 30 days while your project is in development. Once your project is complete, all unused images must be deleted from your computer or other storage devices.