The Exact Moment Each Person Became Assimilated In The Thing

Well, you can't get a more concrete answer to "when was Bennings assimilated" than literally seeing the man getting pulled into the disgusting folds of The Thing.

What's interesting to note here is that we see Bennings without a top on, which proves that assimilation does indeed rip through clothing, meaning that any wardrobe changes from this point on should be taken pretty seriously.

Also, it's in this scene that we get the answer to who sabotaged the blood supply, as Windows in his shock drops the keys to the cabinet as revealed through an audio cue. The Thing in the form of Palmer, Norris, or even one of the dogs, simply takes the keys, slashes the blood, and then deposits them back within Garry's belongings in order to sow the seeds of doubt further.