How long can lithium ion batteries hold their charge?

Depends and depends on a number of factors.

Rate of cycling. Generally the faster you charge a lithium battery, the shorter the lifespan
Depth of charge and depth of discharge. Generally a battery kept in the range of 20–80% will last longer than one charges from 0–100%. Generally.
Battery chemistry. Your cell phone battery even if kept and mimicked the charging of let us say a Tesla (not the original roadster), it will not last as long as a Tesla EV battery. It is because the battery chemistry they use in your cell phone is cheap, energy dense and “good enough”. In an EV it does not have to be as energy dense, but has to last. So the lithium batteries have a nickel or iron backbone where as the cell phone batteries have a cobalt backbone.
If you wanted to get a lithium battery that lasts a long time, lithium titanate is the way to go, BUT they are heavier than others- and costly. Lithium Iron phosphates are up there too in terms of longevity and not as costly, but not as energy dense as others such as NCA, LCO or NCM..

You can do treatments to NCA, NCM and LCO.. But it pushes up the cost to make them last longer.. And example is coating the electrode with gold.. Which in that case.. May be more effective to use the titantes OR the iron phosphates.