Ellie
The older the evidence the less the affect it will have, but it also depends on the reasons why you have been discharged, for example if they have said that there is nothing more that they can do or that your current treatment has resulted in a stable condition albeit with limitations, then it may not matter.
It might be worth your asking to see your medical records at your GP practice they may offer evidence that you have not previously been aware of.
Requesting your medical records Charging for medical records I am not aware of any statistics as to the number of ATOS recommendations made on what is called Scrutiny, the form and supplied evidence, but we do regularly see forum members getting awards without an assessment being carried out.
Lastly, it is important to remember that it is not the condition that results in an ESA award but the limitations that result.
Gordon