朗文词典把the chance / experience etc of a lifetime 作为固定搭配看待,意思是 the best opportunity, experience etc that you will ever have,即:千载难逢(终生难得)的机遇/经历等
the chance of a lifetime 千载难逢的机会 (摘自《外研社英汉多功能词典》)
the holiday / experience /chance etc of a lifetime 意思是 the best holiday / experience /chance etc that you will ever have 百年难遇的假日 / 终生难得的经历/千载难逢的机会等 (摘自《麦克米伦高阶英汉双解词典》)