After Cronos Station mission all other quests are no longer valid considering all the Mass Relays in the Galaxy are shut down except the one leading to Earth.
Missions will remain available throughout the game as long as they don't have a specific time limit. Grissom Academy and Tuchanka: Bomb have a three mission limit, as well as most of the pre-Citadel II side quests, but Turian Platoon and Attican Traverse: The Rachni can be done at any time (strange in the former case since the following mission is timed). In those two cases, you have to unlock them from Primarch Victus and Wrex before they leave the Normandy, and then they'll be available for the rest of the game. RShepard227 (talk)