It used to exist the TickMeter class which for some reason I cannot find now somewhere in order to be able to include it in the source code (if someone knows what is the issue with it at the moment, I think that it would be nice to give us his lights). As you can see with this class there are the methods of .start() and .end(). So in your case what you could do, it would be to create an object of this class TickMeter timer; and when you would for the first time detect the object to start it timer.start(); detect/track the object through frames and when the object disappears stop it with timer.end(); and then you could just call one of the other methods, .getTimeMicro(), .getTimeMilli() or .getTimeSec() in order to obtain the time passed in microseconds, milliseconds or seconds respectively and I think that is what you need.
TickMeter timer; // initialize timer
timer.start(); // start timer when you first detect the object
timer.end(); // end timer when the object disappears
timer.getTimeSec(); // get time in seconds or whatever format you want
If for some reason we cannot use this function anymore you can include the #include <ctime> header instead and apply something similar like:
time_t timer_begin,timer_end;
time ( &timer_begin ); // start timer when you first detect the object
time ( &timer_end ); // end timer when the object disappears
double secondsElapsed = difftime ( timer_end,timer_begin ); // get time in seconds, if you want it in other format you will need to make the correspondent calculations
and another alternative is the std::clock() function again of the ctime header.
const clock_t begin_time = clock(); // start timer when you first detect the object
std::cout << float( clock () - begin_time ) / CLOCKS_PER_SEC; // get time when the object disappears
begin_time = clock(); // you will need to re-initialize your timer though
