Smoothies with frozen ingredients are the hardest to store, as the ice or frozen fruit melts rapidly (even in the fridge), changing the texture and diluting the smoothie within an hour or less. However, if you don't mind the change in flavour, there's no reason not to drink a melted smoothie.
Smoothies With Bananas
Many smoothies are made with bananas, and bananas degrade more quickly than almost any other food. Within a day, cut bananas may brown and soften, and if your smoothie contains a lot of banana, you may notice a change in colour and texture. Bananas actually become sweeter as they brown, so this isn't necessarily a problem. It's safe to drink a smoothie with discoloured bananas.
Other fruits and ingredients that are often included in smoothies may keep for several days in an air tight container in the fridge. There is some air inside most air tight containers, so for best results, place a piece of cling film directly on top of the smoothie. Of course, if you make smoothies as a way to use up fruit that is already going soft, your smoothie will not keep as long.
The Bottom Line
In most cases, it's safe to drink a smoothie that has been in the fridge for at least two days, although the texture and flavour may be significantly affected. Of course, if there is any mould or an unusual strong smell, it's too late.
A way to extend the life of your smoothie is to freeze it in an air tight container. This will stop the fruit from going bad,and preserve the frosty element. If you like thick smoothies, partially thaw it, then blend to your preferred consistency. Frozen smoothies should keep for at least a few weeks.