In Norwegian, you usually don't add a number when writing "one hundred", and adding an indefinite article would make no sense to us at all; just imagine how strange it would sound if you added it in front of "ten years" or "five years" instead of "hundred years".
hundre = a/one hundred
to hundre = two hundred
tre hundre = three hundred
I think, using "a" is not logical because in this sample, the meaning is "about hundred years, not precise" if you add an "a", how can it suppose to mean "about hundred years" ? And as i know, "a" is used with singular nouns, not with a plural noun like "years". You can say, for example "et århundre" which means "a century" (I am not Norwegian neither English, somebody can correct me if i am wrong.)