Within 10 days after the close of the period for filing objections, the board shall mail copies of the objections to the claimant or water user against whom the objections are directed and shall notify by registered mail the persons whose rights are significantly affected to appear before it at a time and place specified and at such time and place the objections will be heard. The time specified shall be as soon as practicable but not less than 15 days from the date of the mailing of the notice of hearing.