You're right, making them last forever unless a new Area field is activated seems kinda a bit much. I actually was a bit iffy about that myself but they will definitely be turn limited. I was actually thinking in the sense of the surrounding environment we see in the Pokemon games. Like for example the summit of Mt.Coronet would have had a Mountain area to start with and Surfing Routes would have an Ocean Area Field to start with etc. So that's kind why I opted against the turn limitations.
I was also trying to think logically when it came to the effects. Like how even the tiniest of flames can start an enormous forest fire, thus I raised Fire Type moves Critical Hit Ratio in Forest. Or how in the sky there is no ground thus me making Ground type attacks power 0 in Sky. Or how in a forest bugs and plants can easily blend into the surroundings thus raising their Evasion in Forest. Or how metal rusts and weakens in water thus lowering Steel types defenses in Ocean. Also, How water Pokemon would be more agile and quicker in water thus the Speed raise in Ocean
You do make a good point though, it would be a bit odd to see a grassland field transform into a massive ocean lol. Also, it makes you wonder how would a Pokemon be capable of doing that in the first place. Well maybe Kyogre and Groudon can but that's about it.