The QGIS task manager widget (shown in the status bar whenever a background task is running) now shows an estimated time remaining and completion time for long-running background tasks. This ETA is calculated using a simple linear interpolation based on the task's elapsed time and current progress, and allows users to know whether they've only got time to make an instance coffee in the office kitchen or walk round the block for a proper barista made coffee before that long processing task completes...
Additionally, QGIS uses the standard operating system progress reporting mechanisms on more platforms and occasions in QGIS 3.4. Some long running tasks (which don't run in the background) such as atlas exports now show their progress via the operating system mechanism, and throw up handy completion notifications.