The announcements technically apply only to G Suite users — that is, people who have paid accounts through their company or organization — and Google declined to comment on whether the changes will be the same for regular Gmail users. But chances are they’ll be similar: the new design was announced first for G Suite users, but it was made available to free users at the same time.
Right now, no one is required to use the new design: it’s either opt-in for you, or it’s unavailable because your G Suite account’s administrator hasn’t enabled it. Starting in July, administrators will be required to give people the ability to opt-in to the new Gmail design; at the latest, the option will be enabled in August.
Some time in September, everyone will be switched over to the new Gmail design. The option to opt-out will still be available for another month. But at some point in October, the option will disappear and G Suite accounts will be required to use the new design.
Basically, Google is giving G Suite administrators several months of warning to prepare themselves — and their users — for the coming change. It wouldn’t be surprising to see Google start to migrate free users over to the new design in the coming months as well. While it hasn’t announced anything on that front just yet, we know that for at least one group of users, the old design will be done for in about four months.