So this is a bit of a boring post however its a very important issue, and it hygiene. Im just going go through makeup items and how long they typically last but doesnt mean it will last that long, some may last longer some may last shorter.
Liquid Foundation - There are two different types of liquid foundation oil-based and water-based, tyically these last 18 months & 12 months, from when you first open them. Please watch or for separation or a change in colour, smell or texture as these are also signs its time to say good bye.
Concealer - This will last you around 12 months, though if the texture becomes cakey on application its time to toss it.
Pressed Powder - Around a year is used with a sponge because the oils are transferred into the powder, but up to 18 months if used with a CLEAN brush.
Loose Powder - Up to two years
Eye Shadow - Will last for up to 3 years, but crumbling eyeshadow is a sign its starting to turn and you should treat yourself to some nice new ones.
Eye Liner - Again this will last you 3 years if sharpened or cleaned regularly, but if it becomes dry and tugs on application you needa new one!
Mascara - This lasts for about four months, but use your own judgement, you should through it away if it becomes dry and flakey.
Lipstick - Between 1 - 2 years, you'll know its off if the smell changes!
Lipgloss - Same as your lipstick, its the colour changes or the smell changes new one!
Nail Polish - When the yellowish layer appears its time to chuck it, but normally around 12 months.
Makeup Brushes - These will last forever is cared for well, but clean after every liquid use and for powder items every week will do.
Other tips, if you want to be really good take a black marker and write the day it was first opened and when it should be chucked that way itll remind you.
Throw away any makeup used if you develope an infection if the bacteria gets into it the infection will only keep re appearing.
Keep makeup out of sunlight, the light and heat will degrade the perservatives, meaning it wont last aslong.
Sorry if that bores you but its always helpful to be reminded, we all love our makeup and wouldnt want to ruin it!