bl_info = {
    "name": "Modal Timer Add-on",
    "blender": (2, 80, 0),
    "category": "Object",
}

import bpy

class ModalTimerOperator(bpy.types.Operator):
    bl_idname = "wm.modal_timer_operator"
    bl_label = "Modal Timer Operator"

    _timer = None

    def modal(self, context, event):
        if event.type == 'TIMER':
            print("Running Modal...")
        return {'PASS_THROUGH'}

    def execute(self, context):
        wm = context.window_manager
        self._timer = wm.event_timer_add(0.5, window=context.window)
        wm.modal_handler_add(self)
        return {'RUNNING_MODAL'}

    def cancel(self, context):
        wm = context.window_manager
        wm.event_timer_remove(self._timer)

@bpy.app.handlers.persistent
def load_post_func(scene):
    print("Load Post")
    bpy.ops.wm.modal_timer_operator()
    
def register():
    bpy.utils.register_class(ModalTimerOperator)
    bpy.app.handlers.load_post.append(load_post_func)

def unregister():
    bpy.utils.unregister_class(ModalTimerOperator)
    bpy.app.handlers.load_post.remove(load_post_func)