The Hedera Governing Council will determine when the Hedera ecosystem has reached a minimum viable set of integrations to enable staking rewards. Once this is determined, the council (through CoinCom) will vote to update the reward rate, and subsequently, the mainnet will be updated with the agreed-upon reward rate.
Once updated, the staking reward account (0.0.800) will be eligible to distribute rewards earned by stakers, once the rewards threshold of 250M total hbars has been met. Rewards will continue to be distributed even if, after this time, the balance of account 0.0.800 goes below 250M.