Give "ProgressBar" for widget addon's etc.

We have UILayout.progress() , it was added in 4.0: #109882 .

Usage Example:

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):
        [a.tag_redraw() for a in context.screen.areas]
        if self._timer.time_duration > 3:
            context.window_manager.progress = 1
            return {'FINISHED'}
        context.window_manager.progress = self._timer.time_duration / 3
        return {'PASS_THROUGH'}

    def execute(self, context):
        wm = context.window_manager
        self._timer = wm.event_timer_add(0.1, window=context.window)
        wm.modal_handler_add(self)
        return {'RUNNING_MODAL'}


def progress_bar(self, context):
    row = self.layout.row()
    row.progress(
        factor=context.window_manager.progress,
        type="BAR",
        text="Operation in progress..." if context.window_manager.progress < 1 else "Operation Finished !"
    )
    row.scale_x = 2


def register():
    bpy.types.WindowManager.progress = bpy.props.FloatProperty()
    bpy.utils.register_class(ModalTimerOperator)
    bpy.types.TEXT_HT_header.append(progress_bar)


if __name__ == "__main__":
    register()
    bpy.ops.wm.modal_timer_operator()

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Source: Using new layout.progress() function in 4.0 - Coding / Python Support - Blender Artists Community

Before that we could show progress under the mouse cursor using WindowManager.progress_*

Usage example:

import bpy, time
wm = bpy.context.window_manager

tot = 100
wm.progress_begin(0, tot)
for i in range(tot):
    wm.progress_update(i)
    time.sleep(0.03)
wm.progress_end()

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Source: python - How to show to the user a progression in a script? - Blender Stack Exchange