According to the law (i.e., FA or Mine Act or BOCW Act and similar enactments on OSH, including the new code), it is stated to employ such a number of Safety Officers as may be specified, outlining their duties, qualifications, and conditions of service. To my knowledge, there is nothing in the rules regarding the time limit within which you should fill up the vacancy of a safety officer. In my view, you should not keep this appointment vacant for any period and should fill it at the earliest possible time justifiably.
Thank you.