David,
As Richard mentioned, there are many different considerations. Anyone that will give you a time estimate without knowing a lot more details (not just about your GP, but about your environment, etc.) would be simply guessing.
That said, if everything in being done on the same environment (ie, you're not moving servers or upgrading to a new version of SQL at the same time), and nothing has changed in your GP setup or data size, I would expect the GP 2010 to GP 2013 upgrade to take about the same amount of time as GP 10.0 to GP 2010 did.