I have found that the turn numbers for a ME seem to feel right at about 35-40 (Variable). As stated before it is more then about flag capture and long extended firefights. I try to time the engagement to begin later, when my units are in place, and then strike hard, push the opposition back and take the victory flags that are then exposed. Prepare for a short counterattack by the opposition and we're done. I think that in context, a Meeting Engagement is just that, a quick decisive battle of oppertunity, not a drawn out affair. That being said, to play on some larger maps and be able to use the terrain effectively does require more time then 25-30 turns.