When will ETH become proof of stake?

The best current (non-official) estimate is sometime in Q2 2022. Long Answer: At the time of writing there is no definitive date for "The Merge". The Ethereum community is pushing for all effort to be focused on the switch to proof-of-stake (PoS).

When will Ethereum switch to POS?

The Ethereum community is pushing for all effort to be focused on the switch to proof-of-stake (PoS). The Ethereum PoS chain is currently running and underwent its first hard fork (Codenamed Altair) in October 2021. For more info about development progress go here https://github.com/ethereum/pm/blob/master/Merge/mainnet-readiness.md

Should I stake my ETH on Coinbase?

There is no staking minimum to stake Ethereum tokens on Coinbase. To stake your Ethereum tokens as an independent validator node, you need 32 Ether tokens. Coinbase aggregates investors’ tokens to run nodes, and it takes 25% of the interest you earn as an administrative fee.