Patents remain valid for 20 years from the date of filing provided they are renewed every year. Further, validity of patents may be questioned by means of post grant opposition (till 1-yr after grant) or revocation by interested parties. If the Patent is revoked, it ceases to exist.
A valid patent is the one which satisfies the three requirements of novelty, inventiveness and industrial application. Taking an example for novelty, if a prior art is discovered which is similar to the present invention then the granted patent may not be able to hold ground during enforcement in Courts.