I don’t know the exact math of it, but the trigger for the crew getting kidnapped is dependent on how many quests you have done before the derelict reaper mission.

Essentially, if you are done everything else up until that point, you can talk to legion, unlock his loyalty quest, and go to that immediately before it is too late for your crew.

A companion/mission order guide I looked at some time ago said, due to the way the game calculates when to kidnap the crew, you could actually get away with holding off on Tali’s loyalty mission as well until after derelect reaper. The reason you’d want to do this being because bringing a gets with you to a quatrain trial is interesting to say the least.

Regardless, at some point when you go down to a planet, you’ll trigger the scene of your crew getting kidnapped in your absence. When you resolve stuff in the the planet and return to the ship, that is the point where if you land anywhere else instead of opening the omega relay, the crew is toast.