An EPC is valid for 10 years.
However if there have been changes made to the property that would make a significant chance to its energy effiicency. You would need a new EPC.
So for instance if you have added an extension, loft conversion or conservatory. This would have an impact on the effiency and trigger the need for a new EPC.
Also if you have added a new boiler or installed new loft or cavity wall insulation. This would also require a new EPC.