many's the…
(formal) used to show that sth happens often 许多次;常常
Many's the time I heard her use those words. 我不只一次听她说过那样的话。
——《牛津高阶英汉双解词典》
many's the formal + literary
— used to say that something is common or has happened often许多次,常常(用于指有些事情很普通或经常发生)
Many's the day we have spent together. [=we have spent many days together] 我们在一起度过了许多天。
Many's the time I've been there. [=I've been there many times] 我去过那儿很多次了。
——《韦氏高阶双解英汉词典》
many’s the time/day etc (that)
used for saying that something has happened often in the past 有许多时候/日子等
Many’s the night I’ve walked back late from town.有许多个夜晚我很迟才从城里走回来。
——《麦克米伦高阶英汉双解词典》
个人看法
《现代英语语法》P.1425,强调表语或为了密切上下文的关系,采用完全倒装。
正常语序是:The time I ate at that restaurant was many.
抱歉,以上为错误看法
Many's [or Many is] the time 曾屡次做过;多次 (very often) Many's the time I have seen him at the station. 我曾屡次在车站遇见过他。(《英汉多功能词典》新编升级版p.1030)
many's the formal + literary - used to say that something is common or happened often 许多次,常常 (用于指有些事情很普遍或经常发生)Many's the day we have spent together. [= we have spent many days together] 我们在一起度过了许多天。Many's the time I've been there . [=I've been there many times] 我去过那儿很多次了。(《韦氏高阶英汉双解词典》p. 1279)
many's the used to show something has happened many times or for long periods of time 多次;长时间:Many's the hour I've spent by the telephonejust waitingin case he should call. 我在电话机旁守候多时,以防万一他打电话来。(《剑桥高阶英汉双解词典》p.1339)