How long can be stringstream be?

Working with a big project in Visual Studio C++11.

To avoid latency I am trying to write the logs of a function(which can be huge and can go beyond 300000+ log lines since the function does some extensive calculations) into a std::stringstream and finally flushing it out to a file.

Is what I am trying to do a bad thing? Is there something I can do better? This is since I dont want to flush it out to a file everytime I want to write a log.

Tia!