Hi i,
ESA is not awarded for a set length of time, it is an open-ended benefit that can be reassessed at any stage once 3 months has passed since the decision was made. However, DWP do document the 'prognosis' which indicates when reassessment can be expected. If you phone DWP this should be on your computer screen and they will tell you. Doing so will not put your current award at any risk.
Nothing on this board constitutes legal advice - always consult a professional about specific problems