1. 钟头 is Cantonese way of saying 小时. 钟=钟头 e.g. 两个钟=两个钟头=两个小时=two hours. 2. In places outside Guangdong, Hongkong and Macow, people just say 小时. 3. 小时 is OK in both Cantonese and Mandarin.

小时用于书面语和口语。钟头只用于口语。

钟头和小时这两个词汇是一样的意思，小时是一个比较通用的一个词汇，但钟头呢，可能几十年前在我们的口语交流中流行，不过现在我们会使用小时会更多些。

You can say "几个小时 "or"几小时 "or"几个钟头 " but you can't say " 几钟头" . For example:2个小时、2小时、2个钟头 are right，but 2钟头 is wrong.

一个钟头=一个小时，but 小时 is standard mandarin while 钟头 is more common in southern China, especially Guang Dong(Cantonese) area.