You can't quantify this effect so well. How it will work out depends on many conditions.
The $50 billion investment is huge.
This graph shows FDI net inflow as a percentage of GDP, it has been pathetically low since 2009. No good at all especially for a developing economy. $50 billion over even 20 years would raise the net inflow compared to GDP above 1% for the first few years only, when Pakistan's GDP increases this ratio will decrease.
And this is one way in which we have growth, year on year, the FDI of the previous years, the benefits still circulate in the economy, that's partly to do with the multiplier effect mentioned above.
But FDI alone isn't enough, it is not a magic cure. It's incredibly helpful, but there a few major milestones before we have the capacity to grow; energy crisis, law and order, terrorism/security, political stability, most important is the last one.
After that, then you might start seeing stable growth if the right policies are used. With the current situation it is exceedingly difficult to have growth without the right conditions.