Unfortunately, we can't provide you with a one-size-fits-all answer to this question. Firstly, the paint type you’re using already plays an important role in storage duration (e.g. water-based wall paint is different from solvent-based topcoat). Secondly, you will also have to take into account whether the can of paint has already been opened or not and whether or not you properly sealed it, as well as all sorts of other important storage conditions (like the temperature in which the paint is stored, for example). Depending on all these factors, you will be able to keep your leftover paint for a few days, weeks, months or even a few years.
Some important tips for storing leftover paint
If you store the paint in its original container, make sure to always close the lid tightly. In between painting, try to make sure you close the paint can as often as possible too. The less air gets to the paint, the better. If you can, try pouring the paint leftovers into a smaller container, such as a jam pot (just make sure that you can close its lid tightly!).
Do not store your paint in places where temperatures are too low or too high. Do not put your paint in the garage in winter, for example, as the temperature is often too low there and your paint will freeze. Paint that has been frozen can no longer be used, so it is of big importance that you keep storage temperature in mind!
How to tell if your paint is still usable
Water-based paint stinks when it has gone bad, so just take off the lid and you will quickly find out whether or not the product can still be used!
After a while, the binding agents in paint products can start flowing to the top of the paint. Should this be the case, make sure to stir the product well before use. If after stirring, you can reobtain the desired consistency, you can safely continue to use the paint. If not, the paint has gone bad and should no longer be used.