Edit: Update Rollup 18 is now available for Exchange 2010 SP3 see the post Exchange 2010 SP3 Update Rollup 18 installation tips.
On 15th December Microsoft released the Exchange 2010 SP3 Update Rollup 12.
I installed Update Rollup 12 on my test SBS 2011 server this evening and it took 28 minutes from start to finish, so I wouldn’t expect it to take much longer than this in production.
The new update was released today so please test before installing into your production environment. Maybe even wait a few days or a week or two to make sure the QA is up to scratch on it.
For migrations to Exchange 2016, Exchange 2010 SP3 Update Rollup 11 is the minimum version that will be supported in a coexistance environment.