Cretae class named "RoleScheduler" and add the below code to that class.
After building the module, add this scheduler class in dotnetnuke scheduler by going to Host -> Schedule -> Add Item to Schedule

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using DotNetNuke.Services.Scheduling;
using DotNetNuke.Entities.Tabs;
using DotNetNuke.Common.Utilities;
using System.Data;
using DotNetNuke.Entities.Portals;
using DotNetNuke.Security.Roles;
using DotNetNuke;
namespace YourCompany.Modules.ModuleName
{
    public class RoleScheduler : SchedulerClient
    {
        public RoleScheduler(ScheduleHistoryItem oItem) : base()
        {
            this.ScheduleHistoryItem = oItem;
        }

        public override void DoWork()
        {
            try
            {
                //Perform required items for logging
                this.Progressing();
               
                // Adds and deletes user role
                AddDeleteUserRole();

                this.ScheduleHistoryItem.Succeeded = true;
                this.ScheduleHistoryItem.AddLogNote("User role has been added and   deleted successfully ");
            }
            catch (Exception exc)
            {
                this.ScheduleHistoryItem.Succeeded = false;
                this.ScheduleHistoryItem.AddLogNote("Operation failed: " + exc.Message);
                this.Errored(ref exc);
            }
            }
        }

        ///

        ///  Adds and deletes the user role
        ///

        public void AddDeleteUserRole()
        {           
                DateTime dtmExpDate = DateTime.Now;               
                PortalSettings objPortalSettings = new PortalSettings();
                RoleInfo objRoleInfo = new RoleInfo();
                RoleController objRoleController = new RoleController();
                objRoleController.DeleteUserRole(objPortalSettings.PortalId,
                                                                                objPortalSettings.UserId,
                                                                                objRoleInfo.RoleID);
                objRoleController.AddUserRole(objPortalSettings.PortalId,
                                                                            objPortalSettings.UserId,
                                                                            objRoleInfo.RoleID, dtmExpDate);         
        }
    }
}

 

Hope this will help you.

Thanks,
Avinash Kumar [ DNN Developer, Mindfire Solutions,India ]