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This is the approach used by a large and very well-known library organization for their Solr installation, because getting access to the source databases for the individual entities within the organization is very difficult. This way they can reindex the online Solr at any time without having to get special permission from all those entities. When they index new content, it goes into a copy of Solr configured for storage only, not in-depth searching.
In this scenario you 'll may have to re-index *twice* in order to utilize that method. Once to your intermediate Solr server(s), then from there to your server(s) that you're using for search.
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Under any circumstance, reindexing will take at least as long as the original indexing took.