What does 头 mean in 时常一待就几个钟头?
As the comments have pointed out, 鐘頭 is a colloquial word which means hour, used in Cantonese, Mandarin, and Wu (e.g. Shanghainese). Other topolects may have their own colloquialisms, the most prominent being 點鐘 (used in Hakka, Min Dong and Min Nan). Mandarin also uses the word 點鐘, but more commonly for the meaning o'clock rather than hour.

The word 鐘頭 comes from an old word for hour, 鐘點 [1], where 點 refers to the hourly divisions in a day. 點 was replaced with 頭, which is a diminutive suffix. For reference, similar words which use this suffix:


木頭
老頭


The word 鐘頭 was likely popularised by modern Chinese literature, e.g. in the work 《日出》 by 曹禺, which was written in the 1930s.


  曹禺　《日出ㆍ第四幕》： “那就看您這幾個鐘頭的本事吧。 我福升實在不能再替你擋這門賬了。” 





李六如　《六十年的變遷ㆍ第二卷ㆍ第九章》： “‘錢就少啦。 ’ 方維夏 把中學待遇略微講一點。 ‘也是按鐘點計算。 公立的一塊錢一點鐘， 私立的八角。 ’”