While hiccups are normal and familiar to most people, and only usually last a short time, it should be investigated if they last longer than a month or so, when it is much less common as there could be an underlying cause. Gastrointestinal causes of hiccups include GORD (gastro oesophageal reflux disorder), hiatus hernia and oesophageal cancer. 

If hiccupping lasts longer than two days and is linked to other issues, such as heartburn or trouble swallowing you should consult your GP as soon as possible for further examination.

References:

Heartburn & Hiccups: Why These Symptoms Could Be Something More Serious Than A Casual Annoyance - Welcome to OGCancerNIWelcome to OGCancerNI