How long can I store diclofenac?

It depends on it's expiry date, issued by the manufacturer. If you have the original packet then it should have it on it. If is was dispensed in another bottle/box and you don't have the batch number or expiry date then it's really hard to say.

If it is a recent dispensing then you could call the pharmacy which dispensed it and they may be able to give an indication of he expiry date.

It should be discarded once it's past it's expiry date