It depends on several factors. The quick answer is that you will need to benchmark each differnt program to understand where that quiescence watermark is.
Here are some of the relevant aspects that could impact that benefit ratio:
1) Computational Dependencies: if the logic flow has many dependencies to external resources ( DBMS, disk access, networking ). The higher the amount of computational dependencies that are divisible in concurrent processing, the higher the benefit of adopting a distributed computation platform such as erlang.
2) Logical flow atomicity: if your program has to spend a large amount of computation time on a single sequential synchronous flow control and that cannot be broken down on smaller logical segments of code. The larger is your code atomicity, the less it can be broken down into CPU spreading flows.
3) State Sharing Overhead: the larger the amount of data that has to be distributed across various functions, the higher the overhead the framework requires to simply transmit and receive the state. In other words, if you are send large amounts of data repetitively without a common shared cache area, the benefits will decrease, although this has different approaches depending on the adopted programming patterns.
Therefore, given the vast possibilities and variations based on criteria such as the above, it is not possible to have a common estimate that is acceptable to all scenarios.
