How to set different time per widget in a cloud-watch dashboard

You can click on the widget's menu and edit it, when you edit the widget, you can modify the time range and check the "persist time range" checkbox. Then update your widget, check the display and save your dashboard.

Alternately, you can set it from the dashboard's JSON source code. For example, I have set a widget with a custom time range of latest 1 hour instead of latest 3 hours, my widget has 2 new properties called start and end:

{
            "height": 5,
            "width": 9,
            "y": 0,
            "x": 0,
            "type": "metric",
            "properties": {
                "metrics": [
                    [ { "expression": "…", "id": "e2", "region": "…" } ],
                    (…)
                ],
                "view": "timeSeries",
                "stacked": false,
                "region": "…",
                "stat": "Average",
                "period": 300,
                "title": "…",
                "start": "-PT1H",
                "end": "P0D"
            }
        },