The Apache Infrastructure team has gotten Git migrations down pat. Just ask the Apache Hadoop project, which moved from Subversion to Git in less than 10 days.
Hadoop committer Karthik Kambatla says that getting from the request to Infra to a usable repo “took less than 10 days. The actual migration itself took 3-4 days. Daniel (Gruno, an Infrastructure team member) smartly suggested we start the process on a weekend to minimize disruption… he updated us fairly frequently on the status and made useful suggestions. There were a couple of follow-up items that were fixed fairly quickly as well.”