How to say "prolonged" in Japanese

• 息の長いいきのながい
  i-adjectiveCovers 'prolonged' mainly in the sense of sustained, long-lasting work or effort. It frames the duration as admirable and enduring, so it fits praising a career, undertaking, or activity rather than neutral description, and it is the less common choice.
  息いきの長ながい仕事しごとになりそうだ。↗

  This is going to require a long steady effort.
• 長いながいJLPT N5approx.Common
  i-adjectiveThe common, default word for 'prolonged' in the plain sense of lasting a long time, e.g. a protracted period or delay. It is purely descriptive of duration with no evaluative nuance, and is usually the word you want.
  株式市場かぶしきしじょうは長ながい不振ふしんを続つづけている。↗

  The stock market is in a prolonged slump.

For most everyday senses of 'prolonged' use 「長い」; reserve 「息の長い」 for praising something like work or effort that endures over a long span, where it carries an appreciative nuance.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

• 息の長いいきのながいlasting (work)
• 長いながいlong (time)