Of course, your cached coverage data is likely to become invalid at some point. Thus, result sets that are older than SimpleCov.merge_timeout will not be used any more. By default, the timeout is 600 seconds (10 minutes), and you can raise (or lower) it by specifyingSimpleCov.merge_timeout 3600(1 hour), or, inside a configure/start block, with just "merge_timeout 3600".
You can deactivate merging altogether withSimpleCov.use_merging false.