The liquid used in electronic cigarettes have a shelf life of about one year from the date of manufacture. During the first few weeks/months the flavors will go through a period of time called "Steeping" that enhances the flavoring over time. Some flavors will become more pronounced during this period of time as the ingredients used for flavoring become more and more integrated into the Vegetable Glycerin and Propylene Glycol used for the construction of the eJuice.
eJuice and Nicotine
Liquids with Nicotine will become more viscous over time, and the colors will become darker and more pronounced. Liquids with Nicotine naturally have a more pronounced color than their counterparts with smaller amounts to no nicotine.
High VG eJuice
Vegetable Glycerin is a vegetable sugar and is considered a food product. eJuice with a majority amount of Vegetable Glycerin tend to go bad quicker, and will become noticeably less appealing the closer the eJuice is to the one year earmark. It is best to use high VG eJuice sooner than their counterparts with a higher ratio of PG, as Propylene Glycol is simply a moisture holder and has more of a preservative effect. High PG ejuice have been known to be OK to vape after the one year mark after production.