This depends on various factors.
In principle, a card does not open earlier than the cards before it. So if a map is still closed before you, yours will definitely not open soon.
Usually, cards are opened a few weeks after the war on the card has ended and an alliance has emerged as the victor. So the alliance that has emerged as winner has enough time to build itself up and can already attack other cards without having to fear for its own card.
7 days before the card opens a big countdown appears under the logo. You'll be warned soon enough.
Tip: If you build up all bases at the edges and 2-3 rows behind them, the enemy will have an easy game when he breaks through and comes across un-built bases. Invasions often occur at night, so be vigilant.
Note: Game mode free4all. The free4all maps are filled up with yellow bases who everyone can take and open after an countdown to be rushed by all to have fun together. These maps can be between other closed maps.