Hi maurice,
Unlike DLA, ESA is actually an indefinite award that can be reassessed at any time after 3 months has passed since the decision was made. The maximum length is 2 years in WRAg and 3 years in the Support Group. In practice a 'prognosis' is usually entered on the claimant's file and this is the length of the award, reassessment forms being sent out several months in advance.
If you wish you can phone DWP and ask for the prognosis, or write and request the paperwork associated with your claim.
ESA medical – what forms to ask for? The issue is covered in this FAQ
How often will I be reviewed