I posted a proposal for a fixed schedule on the PM tracker: Ethereum Core Devs Meeting 52 Agenda · Issue #66 · ethereum/pm · GitHub
Now, that Constantinople is finalized, I would propose a rough schedule for a subsequent protocol upgrade (a.k.a. “Istanbul”?):
- 2019-01-16 (Wed) projected date for mainnet-hardfork (“Constantinople”)
- 2019-05-17 (Fri) hard deadline to accept proposals for “Istanbul”
- 2019-07-19 (Fri) soft deadline for major client implementations
- 2019-08-14 (Wed) projected date for testnet-hardfork (Ropsten, Görli, or ad-hoc testnet)
- 2019-10-16 (Wed) projected date for mainnet-hardfork (“Istanbul”)
That breaks down to a fixed 9-months cycle to release protocol upgrades accepted prior to the hard deadline in May to mainnet. All proposals accepted after that date should go into a subsequent hardfork nine months later.