Guys my brother and I live in Germany. My parents are in India. We helped our parents prepare the visa documents for short term tourist/family visit. They received their stamped passport within 3 days of visa interview.
I guess Verpflichtungserklärung / obligation letter from my brother who has a PR along with my father's financial documents made a strong case. My father in his cover letter explicitly stated that my parents have no intend of staying longer in Germany. Also my parents have a good travel history. Point is if your documents are good its a matter of 3 days.
Make sure you have all the documents in your check list (insurance, application, bank statments,obligation letter, invitation letter). My father also submitted extra documents like three months of bank statement, fixed deposit, mutual fund investments.
I am so happy :)