20 Minutes Mark

Overview
The 20 Minutes Mark is known for being the time where Command Centers stop doing an astronomical amount of damage, it's also, for many people, where the early-game ends and where the mid-game starts. This is a period in which most of the rushes happen, especially Space Fighters rushes that focus in killing Nuclear and Power Plants, however, these actually need some strategy and an actual teamwork from the entire Alliance to be achieved.

Strategy
This is the time where most players that are hunting for skins will prepare to strike if they aren't doing so already, the NP and PP killing should also intensify. If you want a long game, try to make as many Plants as you can and gather Income while also letting your enemy expand their territory, although, it's important to note that your enemy should never have control over the majority of the territory, it's up to you to control their Income by killing their plants. You could also get a Tank Factory or Airport more soon-ish for defense, in later terms a Space Link would be the best choice.

If you don't want to waste more than 40 minutes playing a game, you can always rely on your Allies to help you get mid-game buildings, like Space Links, at around the 20 or 30 minutes mark. Kill enemy plants before they manage to get enough defense or big armies. However, the Space Fighter management will have to be done by you as it changes from player to player, game to game.