At present, if you're entitled to contributions-based ESA, you continue to get it without time-limit.
However, the government has said that it will bring in a time limit of twelve months for people in the WRAG, after which you will have to transfer to income-based ESA if you are eligible for it on income and capital grounds, otherwise, you will get nothing.
If you are in the support group, the postion will continue as at present, namely that there will be no time limit for contributory ESA.