How long do you keep your makeup

Most of my MMU is life time so I never toss it. IF my L/S have not bad order dip in alcohol ever so often and keep them clean. Eye liners and gels I used up in a year so most the time I do not have to toss it.  

For lipsticks and pencil liners, if there's no bad smell, and you keep them capped they should last you until you finish using them up. When in doubt, dip in alcohol.

For e/s and other powders, those should last you a lifetime so long as you keep the lids closed. 

Liquid liners, I'd be a bit more wary as every time you use them you expose the applicator to air and its particles and then dip it back into the liquidy goop. Generally the lifetime of these is about the same as mascaras (~6 months). Granted, I've kept some of mine for much longer and still use them, but to each their own risk as I'm thoroughly aware of mine. However, you said purging, and if this is stuff you're considering to sell, then if it's older than 6 months I wouldn't sell it.  (And in general, if you've personally used the liquid liners without a new and clean applicator, I wouldn't suggest selling it to others. What may be natural flora for you may be harmful to someone else.)

For Liquid foundations max I will keep is 1.5yrs

Powder n blushers I will keep for 5 or 6 yrs, and many times they last much longer

Cream blushers, I will keep max 2 - 3 yrs and I will toss it out.

Some of my lipsticks are really old, but they are still pretty new and there's no change in smell or color so I still keep

I keep everything for about a year. After a year, I throw it out.

Powders last forever. They can always be sanitized with a spritz of alchohol isopropyl. To be quite honest, I don't throw away my lipsticks even if they do have a bit of an odor - and I've never had much of a problem. Eye pencils can always  be sanitized as well and you can keep liquid liners until they dry out.

anything powdered, I keep unless I just don't want it. e.g. powder blushes, powder foundation, eyeshadow, etc. I still have a loose powder foundation I bought 5 years ago, so that's probably my oldest makeup item.

lip sticks, maybe a year or two depending on how often I use it (I have a bunch of cheap NYX ones I've only swatched like once).

If eyeliners become dry and scratchy (gel or pencil) I'll toss them. I don't really use liquids, and when I do I use felt-tip ones.

Lip glosses are the only things I'll toss out pretty frequently, probably since I have so many from sample subs :P.

Check this MuT article out! This might help :)

I have one stick bronzer from Origins that I've had for almost 10 years. It has never smelled and the consistency has never changed. I've never had issues with it. That would be my oldest thing probably. I throw out glosses when they start to separate. I don't think I've ever thrown out an eyeshadow unless I just didn't like it. I'm pretty sanitary with all of my stuff. I only apply mascara in the mornings and don't reapply throughout the day. I think this drastically cuts down on bacterial issues. I've never gotten any eye infections, so my system has worked quite well for me. I think it all just depends on personal taste. If it bugs you to keep something for more than the "recommended" time, then throw it out. Otherwise, I think if you are clean and sanitary, then you shouldn't have any issues with your collection.