Knowing the 'best-before period once opened' is of paramount importance for consumers, because it gives information about how long a given food keeps its quality. The hydroxymethylfurfural (HMF) content is related to honey's aging and heating. We researched the evolution of HMF of 60 unheated honeys originating from continental and oceanic climate areas, recording the moment at which the HMF increase begins to speed up. We also researched the influence of induced granulation on HMF evolution. All samples were stored at room temperature and analysed every 5 months throughout 30 months. Storage had an effect on HMF contents. On the contrary, induced granulation did not show any influence on this parameter. HMF constantly increased in all samples but much more markedly after 20 months. At 20 months, the percentages of samples with HMF higher than 40 mg/kg ranged between 2.9% and 8.0%. Nevertheless, at 25 months these percentages clearly increased, ranging from 22.9% to 72.0%. For honeys from both the continental and oceanic climate areas, taking HMF contents into account, 20 months could be proposed as the 'best-before period once opened'.