Reproduction steps:
1. Open "test2" attached project.
2. Open and run "Assets\Scenes\MainLevel" scene.
3. Click "start" button.
4. Open console window.

Actual behavior: agent.remainingDistance equals infinity when too much enemies are in the scene.
Expected behavior: agent.remainingDistance shows distance to the next waypoint.

Additional notes: to change enemy spawn speed, click on "spawner" game object and change "Spawnpausetime" value.

At low framerate the fixedupdate (used in the project) will do many calls per frame - where agents will be waiting for path request to finish.
See also : http://docs.unity3d.com/ScriptReference/NavMesh-pathfindingIterationsPerFrame.html