In Spanish, the adjective mínimo can be used both as an adjective that agrees with the noun (mínima) or as an adverb (mínimo) when indicating a minimum amount or extent.
In the phrase "una semana mínimo," mínimo is functioning as an adverb, meaning "at least" or "a minimum of." It's similar to saying "one week at a minimum" in English, which is why it doesn't need to agree with the noun semana.
If you were describing something as the "smallest" or "least" of its kind, then you would use the adjective form that agrees with the noun, for example:
una cantidad mínima (a minimum amount)
una duración mínima (a minimum duration)
In summary:
Mínimo (adverb): "una semana mínimo" (one week minimum)
Mínima (adjective): "una semana mínima duración" (a week with minimum duration)