I had a good 20 turns or so before Numidia attacked me (VH/VH). Even then, they had one large stack that started the war by beseiging one of my cities. After this army, I only saw one further large stack in my conquest of the Numidian provinces. All in all, their biggest impact on my was the loss of trade income which I experienced when I went to war with them. Militarily they were never much of a threat.