I know Wiki isn't the end-all be all, but this is what it says:
The NBA has established two draft declaration dates. All players who wish to be drafted, and are not automatically eligible, must declare their eligibility no later than 60 days before the draft. After this date, prospective draftees may attend NBA pre-draft camps and individual team workouts to show off their skills and obtain feedback regarding their draft positions. A player may withdraw his name from consideration from the draft at any time before the final declaration date, which is 10 days before the draft. A player who declares for the draft will lose his college eligibility, even if he is not drafted, if any of the following is true:
- The player signs with an agent.
- The player has declared for and withdrawn from a previous draft. Although the NBA collective bargaining agreement allows a player to withdraw twice,[1] the NCAA only allows one withdrawal.