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?
I only have tiny amount of tBTC and i don't plan to sell those. However,
1. You can try either tBTC faucet or exchange.
2. Depending on your needs, you could setup regtest (testnet on private environment) or obtain signet (newer iteration of testnet with few differences) Bitcoin.