For the theoretical bandwidth, that is not a number you can calculate, it is part of the specifications of the processor. For example looking at the specification page for a Intel® Core™ i7-4980HQ Processor under Memory Specification you can see it states the max bandwidth is 25.6 GB/s.
As for practical throughput, you would have to look at the RAM bottlenecks and see what the throughput of the ram you are using is.