During its recent reveals for Sonic Mania, SEGA have focused on presenting older levels from previous franchises as opposed to new levels and the reason is to maintain surpirse for the players by not giving everything away.
Sonic PR & Social Media Manager Aaron Webber explained during an official live stream that they don’t want to spoil the entire game for players. If some unauthorized spoilers to happen to surface as we get closer to the launch of the game and it would then be up to the individual person whether they want to spoil the game for themselves.
As regards to the length of the game, Webber commented that it will depend on how much the player wants to explore each level and how fast they go, much like the earlier entries in the Sonic franchise, where if you are really good and know all of the secrets and tricks, you can shave off a lot of time from your run. There are also other elements that can extend gameplay but they may be discussed at a later date.