FYI, Opensolr doesn't offer a CPU usage measurement for the Shared OpenSolr Cloud, since, obviously there are multiple users using a single instance or many instances at the same time, in which case, we measure the "stress" of each index, via the Bandwidth, which is actually the number of requests you send to our servers, and size of data that those requests return to your application.
It's basically a measure of the data that our servers are transferring to your application.
If you go ahead and look at https://opensolr.com/pricing you'll notice that the bandwidth is limited to 10 Gb (in the customisation sliders below the predefined packages).
Therefore, if your index exceeds 10 Gb monthly bandwidth, or 5 Gb disk size, you should probably think about upgrading to either of the plans described on our services page: https://opensolr.com/faq/view/pricing/50/What-types-of-Solr-Hosting-do-you-provide
Hope this helps.