It will come back when whatever is using the memory releases it.
If you want to know the details on what is doing it you can use task manager in windows, or what I prefer process hacker because it provides more details.
download process hacker here:
http://processhacker.sourceforge.net/
Then run it and look at the processes tab and see what is consuming the most memory. Right click that process that is using the most memory and pick properties to see what it is using, what it has paged, etc.
Example , firefox looks like this on my system:
Private bytes is memory no other application can get till I close firefox
Working set is the private memory + caching memory so about 13MB that an application can get if firefox is not in focus.
Peak working set is what firefox used at its most .
Virtual size is the combination of physical memory + memory paged out to the hard drive.
That should help you find the source of the memory usage.
