The shelf life on tile adhesives is usually around 6-12 months, depending on the type of adhesive you are using (ready-mixed or powder). It is of course always best to go by the manufacturers’ recommendations as if anything goes wrong, you won’t be covered by any form of guarantee. However, as with anything, if the use-by date is 30 June, it will not suddenly be unusable at 1am on 1 July. So, if you are willing to take the risk you can use your best judgement on this.
The reason adhesive has a shelf life is because it might begin to set over time (whether opened or not). If you are using powder it might begin to set due to the small amounts of moisture getting in. If you are using ready mixed it might begin to set due to the small amounts of air getting to it.
As a general rule, if there are no hard lumps in the adhesive it should be OK to use. With that in mind, expired adhesive may harden too quickly once mixed (powder), be too dry to spread (ready mixed) or in some cases do not harden at all if the setting agent has expired.
If anything does go wrong with your application then you will need to remove it all and start again (with new adhesive) anyway – so it is probably best to stick to the guidelines and if you see your adhesive is due to expire soon, use it before it does.