Can anyone share their experience using digital twins? How long did it take to implement and how are you currently using them?

I have used different vendor's digital twins (from equipment OEM and 3rd party); depending on the requirements, maturity level of IIoT and what the goal of the exercise is (connecting twins is very difficult) so I would estimate 4 months on the low end and 1 year at the high end. 

Depends on the type of twin, data sources and outcomes.
I visual twin where you see pretty 3d objects and real-time data will be a couple of months with a full team and if your data sources aren't complicated or legacy. 
But if you're looking at extracting insights by doing simulation / scenarios or full command and control automation to adjust operating environment to fall in line with optimal scenario probably looking at over 12 months

Implementation time would depend on the landscape and depth of integration with sensors. Typically 8-12 months is sufficient for current generation of IOT sensors and solutions. Legacy systems might take more time and would depend on internal skills and OEM support