SEI has collected some data from different organizations for different maturity levels and has observed that on average a 20 month time was required by all organizations achieving CMMI Maturity Level 3 from CMMI Maturity Level 2.
Time duration for completing a Maturity Level target depends on following factors:
- Current Processes
- Implementation Status
- Compliance Level
- Cultural Issues
But again this time can be lesser or more depending on the actual implementation status at the organization and the targeted maturity level.