Good point. The length could be adjusted by map size. Small & duel maps, -1 turn, large & huge maps +1 turn ?
Once i played on huge map with 2 continents, the baseraph ai went rampage (on prince!?)and conquered 2 other ais and then turned on me with 3 stacks 12+ units of expierienced swordsmen catapults priests. I only had some archers, some swordsmen and cavalry at that time. So i was outnumbered 5:1 and had no chance to hold my citys. I casted the ljosfalar worldspell and thought hey i get 3 or 4 treants but instead i got about 5-6 per city!? So in total about 40 treants which last 5 turns mmhmm... the first thing i did is i attacked my other neighbour and took 2 citys from him within 2 turns. Treants too slow not cabable to attack a city no? 3 or more 11 strength units against bowmen axemen? Ironically the invasionforce of the baseraph was so big even the treants barely where able to push them back, but my neighbour was overrun by the trees of terror!
I would ask for a change: 1: The treants only spawn around the unit that casted the spell. For each ancient forest tile you get 1 treant but not more than 5 which also last 5 turns. If there is any hostile unit entering your realm the chance of spawning a treant is increased by 50% for 5 turns.