Since 2005 all cosmetic products have had to carry a symbol indicating how long they should be used after opening. It takes the form of a jar with an open lid and a number – 12M or 24M, for instance. This suggests the product should be safe to use for either one year or two.
Liquids and creams, including mascaras, tend to have the shortest lifespans as they're more liable to harbour bacteria. Products with organic or natural ingredients may also be shorter-lasting as they probably won't contain the synthetic preservatives that prolong usage.
If a formulation starts to separate or the smell or texture changes don't take any chances, as it could cause a reaction.
While the symbol tells you how long you can safely use a product after opening it, the problem is that few of us will be able to remember when that was. So a really handy tip, especially if you have lots of beauty products, is to write the date you started to use it on the label as a reminder.