You can run the trip line at least 40 feet. At some point, the weight of the trip wire itself might pull the firing pin out. However, in that case, you can use the "tactical setup" described above.
Pro Tip: to create a circular perimeter, guide the trip line along the perimeter's path by running it through the eye of several eye bolts. With this method, a single trip line can be guided in multiple directions to create a circle.