1. There is no limit to the number of times. This does not mean you will keep on extending. A staff whose performance could not be improved despite all the efforts to guide, to train him. could have been asked to leave long ago.
2. Extension can be given at any time after receiving assessment from his immediate superior.
3. You may discuss the assessment report with him and it is not necessary for to sign anything before issuing the extension or confirmation.
Whatever you do, better .be sure to have the backing of your superior or the management.