Dashboard panels don't really cache information. They run each panels search at the time of the dashboard loading. However, you can schedule a report and import the results of the scheduled report into a dashboard panel. For instance, if you scheduled a report to run once a day at 00:00 then the dashboard would show the results of the scheduled report.
It seems like you are trying to do that now, but you get an error on the number of historical searches. You can fix this error by updating the limits.conf file in $SPLUNK_HOME/etc/system/local/. Which would allow for more historical searches. There is another Splunk answers topic that explains this at: https://answers.splunk.com/answers/54674/how-to-increase-the-maximum-number-of-concurrent-historical-searches.html
However, I personally think that the real issue is how long it takes to conduct a search for a week's worth of data. I would definitely pair down my dashboard to 10 or less panels, then create a second dashboard with the remaining panels. Or just link to the reports in the dashboard for information on the dashboard that is not as critical. You could also post some of your searches here for the community to review if you would like them to help you optimize the search query. This is a much better option than modifying your limits.conf file, since that could have some negative results in your system's performance. With 15 concurrent searches, the server sounds like it might be a little underpowered anyway, and optimizing your searches should make it present your results faster with less wear and tear on your system.
Here is a link to a doc that talks about search optimization: http://docs.splunk.com/Documentation/Splunk/6.2.5/Search/Writebettersearches
... View more