If all goes well, 2022 [0]. The transition will happen in multiple phases. IMHO it's very interesting from a software engineering perspective.
- The Beacon Chain is already live (since December last year). It's going to act as the coordinating entity in the PoS system. Future validators are already staking Ether on it.
- Shard chains support is expected to be launched later this year. The plan is to have 64 shards to improve scalability. Together with Layer 2 solutions (which are on the verge of becoming mainstream, e.g. Jack Dorsey's tweet was minted using a L2) that will bring the throughput to ~100-200k transactions per second.
- The last step is "docking" the old Ethereum Mainnet to the Beacon Chain as one of the shards of the new PoS system.
