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Where can I find the total front-end instance hours that I have used in my previous days? It seems I can only see today's total.

AFAIK the actual instance-hours numbers are not directly available, at least not in the developer console.
What you might find helpful would be the historical graph of the instance usage, for the last (up to) 30 days, which you'll find in the Dashboard after selecting the Instances display mode, the desired timescale and the billed instance estimate graph:
Hover the mouse cursor over the graph and the actual billed instance estimate value will be displayed below the graph and the corresponding date and time is displayed in the top right corner. Note that there is some noticeable delay in updating the values, tho.
The graph detail level depends on the selected timescale, the coarsest being ~2h (on the 30 days timescale), so if you need more than just a general idea about usage trending you'd need to take care of:
averaging and/or integrating on a daily basis
accounting for the free daily quotas
accounting for instance class (I'm unsure if that's already taken into account or not)
You could also try the billing date export feature. From
Export billing data to a file:
You can export your daily usage and cost estimates automatically to a
CSV or JSON file stored in a Google Cloud Storage bucket you
specify. You can then access the data via the Cloud Storage API, CLI
tool, or Google Cloud Platform console
...
Alternatively, you can export detailed data to a Google BigQuery
dataset. For more information, see Export billing data to
BigQuery.
Note: I haven't actually tried this export feature. so I can't tell if the instance hours values are in there. There's also the still open GAE issue 10716 which suggests GAE stats might not be included.