clock_t now = clock();
myStartTime = start;
myTimeLimit = 5 // in seconds

for (int depth = 2; ((double)(now - myStartTime)) / (double)CLOCKS_PER_SEC < myTimeLimit; depth += 2)
{
    //
}

var now = DateTime.Now;
myStartTime = start;
myTimeLimit = 5;

for (int depth = 2; (now - myStartTime).TotalSeconds < myTimeLimit; depth += 2)
{

}