Literally every entry outside of Yellow and Stadium implies over time given the information that rages do not happen instantly.
"They say that during past wars, Gyarados would appear and leave blazing ruins in its wake."
Either implies nothing or implies that Gyarados destroys a city (or not, "ruin" is pretty vague) whilst travelling through it. Ancient cities for that matter.
"Once it appears, it goes on a rampage. It remains enraged until it demolishes everything around it."
Implies that it's rampages are over time and that it can't instantly destroy the things around it.
"It appears whenever there is world conflict, burning down any place it travels through."
Once again implying it destroys places as it travels through them.
"They say that during the past wars, Gyarados would appear and leave blazing ruins in its wake."
Repeat of above entry.
"When Magikarp evolves into Gyarados, its brain cells undergo a structural transformation. It is said that this transformation is to blame for this Pokémon's wildly violent nature."
Says nothing about destruction.
"Once Gyarados goes on a rampage, its ferociously violent blood doesn't calm until it has burned everything down. There are records of this Pokémon's rampages lasting a whole month."
Once more implies it takes time to destroy the things around it. Given that above entries say it calms down once it has destroyed everything in it's area this entry would imply it can take up to a month to destroy a city.
"It is an extremely vicious and violent Pokémon. When humans begin to fight, it will appear and burn everything to the ground with intensely hot flames."
Says nothing of what it is burning but does imply it destroys things via fire, which would mean it most certainly cannot destroy a city with one blast unless its fire breath is several hundred kilometers wide.
"It has an extremely aggressive nature. The Hyper Beam it shoots from its mouth totally incinerates all targets."
NLF entry that is most likely referring to Pokemon anyway.
"Rarely seen in the wild. Huge and vicious, it is capable of destroying entire cities in a rage."
Once more says its city destruction is done through prolonged rages.
"Once it appears, its rage never settles until it has razed the fields and mountains around it."
States again that destroying the things around it is a long process.
"In ancient literature, there is a record of a Gyarados that razed a village when violence flared."
States that it destroyed a village with no mention of timeframe, even more dubious since it's ancient literature.
"Once it begins to rampage, a Gyarados will burn everything down, even in a harsh storm."
Repitition of previous entries that Gyarados rampages until it has destroyed everything around it.
"They say that during past wars, Gyarados would appear and leave blazing ruins in its wake"
Copy of previous entries.
"Once it appears, it goes on a rampage. It remains enraged until it demolishes everything around it."
Copy of previous entries.
"Once it begins to rampage, a Gyarados will burn everything down, even in a harsh storm."
Copy of previous entries.
"In ancient literature, there is a record of a Gyarados that razed a village when violence flared."
Copy of previous entries.
"Rarely seen in the wild. Huge and vicious, it is capable of destroying entire cities in a rage."
Repitition that it destroys cities during it's long rages.
"When Magikarp evolves into Gyarados, its brain cells undergo a structural transformation. It is said that this transformation is to blame for this Pokémon's wildly violent nature."
Says nothing.
"Once Gyarados goes on a rampage, its ferociously violent blood doesn't calm until it has burned everything down. There are records of this Pokémon's rampages lasting a whole month."
Says that it burns things down over time, this lasting even up to a month.
"A tale is told of a town that angered Gyarados. Before the sun rose the next day, flames utterly consumed the town, leaving not a trace behind."
Possibly the village in the ancient literature mentioned previously. Gives a timeframe of less than a day, but still implies it took a while.
"There are people who swear that any place Gyarados appears is fated for destruction."
Says nothing.
tl;dr
There are 6 entries stating it can destroy cities, at maximum. Only 3 of these entires do not mention this taking place during a rage, which have 5 entries implying they take place over time. This timeframe ranging from 1 day to a month.
