I think OP actually want to ask "When will Bitcoin Testnet mining reward (excluding reward from TX fee) reach 0?". Since BTC and tBTC have same method of supply generation, it'll happen on block #6930000. Current block height on testnet is already at #2582202. At current pace and considering people sell/buy tBTC, i expect it'll happen within 10 years or less.
I will buy tBTC do you have any to sell?
Testnet3 was introduced to avoid the problem of high difficulty and limited supply would increase that difficulty.
Testnet coin versions are also periodically changed when too many people mine & hold them and even worse - sell them to idjits who think they have some sort of value. Each new version make the older ones unusable.
As has been said earlier it only takes a small amount of tBTC to test software and in most cases once your testing is done you are supposed to return the tBTC to the sources you got it from which makes limits a moot point.
They have value even if they were meant only for testing. The fact that users need them creates value and demand. without a doubt these testnet tokens will sell for atleast 1% of the main nets price eventually. Maybe even 10%... I think the testnet value can help the mainnet and onboard more users looking to try Bitcoin out before they go in deeper. It'll ease congestion on the main since Ordinals was created. gives a chance for newbies to get in on the action and actual hold some coins instead of fractions of 1. Its like a second chance to people who passed or get distracted.
I'm working on bringing value to the testnet hoping they wont reset this one since its been running for over 10 years now. The testnet should not effect the main in any negative way if Bitcoin is really a solid protocol. No worries just adoption the testnet should never be more important then the main but it should have value. less than 1% of the main net wallets hold most of the Bitcoin. When you got the CEO of Binance saying they have the power to rollback the blockchain is scary. No one should have that much power.