We make green smoothies daily. When my daughter cannot finish hers,
we put it in a mason jar in the refrigerator. She usually drinks
the rest by the end of the day. There have been one or two times,
the smoothie was not drank until the next day.

According to Victoria Boutenko, author of “Green for Life”....

Storage of Green Smoothies

While fresh is best, green smoothies will keep in cool
temperatures for up to three days.

According to Steve Meyerowitz, author of “Juice Fasting &
Detoxification”.....

The following could be applied to smoothies as well as juices.

Fresh is best. Freshly made juice is highly perishable.
Any contact with light, heat and air, even at room temperature,
commences the process of oxidation.

However, ‘cold’ is the key word in storage.
You can make enough juice for the whole day or even a couple
of days if you can keep it cold enough. The object is to keep
the juice as cold as possible without freezing. This means 35
to 38 degrees F. Use a dark, sterile glass bottle that has been
pre-chilled. Fill your juice right up to the top so as to reduce
the presence of air and oxidation. If you keep your jar sealed, it
will stay for about three days. Once you start opening and closing
the jar, pouring from it and replacing it in the refrigerator, the
contact with air, light and heat commences oxidation, which continues,
albeit at a slower rate, even in the refrigerator. The more you open and
close the jar, the faster the juice will deteriorate

Frozen juice is not as preferable as fresh, nevertheless it is another
alternative. The process of freezing and thawing destroys some fragile
vitamins, enzymes, and cell factors. It is not as nearly as destructive
as pasteurization, canning, or irradiating. Frozen juice still taste good
and it is better than buying bottled juices at the store. It is one level
removed from being totally fresh and alive. Freeze only if you have more
juice than you can drink, or if circumstances deem it the best alternative.

The juicing machine plays a role. Not all machines are equal.
Better machines are able to extract more live nutrients, enzymes
and anti-oxidants which act like preservatives and keep the juice
stable and extend its life.

However, I have read it is best to drink your juice within fifteen minutes
of juicing, due to oxidation.