Hi Horty,
ESA is technically an indefinite award, however a reassessment date will be entered on your computer file. You can find out when to expect this by phoning DWP and asking for the 'prognosis'. Forms are usually sent out 2-3 months ahead of this date.
You should treat every reassessment in the same was a new claim, filling in the ESA50 form in detail and submitting copies of all supporting evidence, even if DWP have already seen it.
Some members like to keep a copy of the ESA PDF form up to date to save time and energy when the reassessment does happen.
Using The PDF Version Of The ESA50 Questionnaire