The data in the cool tier is "considered / intended to be stored for 30 days". But this is not a must. You can store data indefinitely in the cool tier. The mentioned reference (see below) even gives an example of large scientific or otherwise large data which is stored for long duration in the cool tier.
https://docs.microsoft.com/en-us/azure/storage/blobs/storage-blob-storage-tiers?tabs=azure-portal