redundancy
It seems to me that "time period" is frequently used in speech and writing. But isn't it redundant?
Not sure if this might be the best example but it seems that the same meaning can be conveyed without using time period.
Usage of conspirator / co-conspirator has changed significantly over the past century or so...
But this applies only when it means others involved in the same conspiracy as an identified subject explicitly referenced. In all other contexts we still overwhelmingly use the "non-redundant" form...
Taking that distinction into account, I don't think co-conspirators is normally a "redundant" form. If I came across your conspirators today, I'd assume the person being addressed wasn't directly involved in the conspiracy at all. He might have been the one who stood to benefit from it, or who "bankrolled" them without knowing the details of the operation. Or perhaps he just suggested there might be a conspiracy...
"You're so cynical! Half the time I don't think your conspirators even exist!"
I would not consider it redundant to have if...then...
Leaving aside the point of using then to clearly mark where the consequent clause begins, the use of both if and then can serve to emphasize the causal nature of the antecedent, or to make it seem like an if and only if rather than just an if-then.
For example:
If it rains, we will stay inside.
merely provides the plan of action in the case that it rains, whereas:
If it rains, then we will stay inside.
seems to suggest that the staying inside will only happen if it rains (note the emphasis on then, which would be stressed in speech and italicized in writing).
Best Answer
While period does refer to calculated time, it also refers to more organic (temporally contiguous) divisions. We reference Picasso's 'Blue Period' as a piece of time associated with when Picasso painted blue things. The length of time associated with the Victorian Period is usually stated as 1837-1901, but this is only one of many attributes of the Victorian Period/Era.
So while you're correct in theory, in practice, we use 'time period' as referring to the numerically quantifiable length of time. This sets it apart from a period's other attributes.