三時間もかかった・
Place も after a quantity to present that amount as surprisingly, unusually, or impressively large for the situation.
The expectation is what matters
The stated number does not have to be objectively large. It only has to exceed what seems normal or expected in that particular situation. Ten people may be a small crowd at a stadium but a surprisingly large line outside a tiny shop.
Typical reactions to a large amount
Each sentence supplies a reason why the quantity feels high.
この仕事は一時間で終わると思っていたのに、三時間もかかった。
I expected this job to take an hour, but it took a full three hours.
Analyze小さな店なのに、開店前から三十人も並んでいた。
Even though it was a small shop, as many as thirty people were already lined up before it opened.
Analyzeこのノート、一冊五千円もしたの？
This notebook cost five thousand yen?
AnalyzeThe surprised tone and the kind of item make the price sound unusually high.
簡単そうな問題だったが、正解するまでに五回もやり直した。
The problem looked easy, but I had to redo it five times before getting it right.
AnalyzeA large reaction versus a minimum
も presents the stated amount as strikingly large. After a quantity, は instead gives a minimum: the amount was at least that much, and it may have been more.
More examples
彼は一人でケーキを三個も食べた。
He ate three whole cakes by himself.
Analyze予約が取れるまで、電話を二十回もかけた。
I called twenty times before I managed to make a reservation.
Analyze彼女はこの本を一日で百ページも読んだ。
She read an impressive hundred pages of this book in one day.
Analyze会議には予定より十五人も多く集まった。
Fifteen more people than expected came to the meeting.
Analyze駅から海岸まで十キロも歩いた。
We walked a full ten kilometers from the station to the coast.
Analyzeこの小さな箱には写真が二百枚も入っている。
This small box contains as many as two hundred photographs.
Analyze