Anything beyond 25 feet doesn't really give enough control in sticky situations. With a longer lead, even a 40 pound dog can pull a human down when they hit the end of the lead at a run.

We made our own check cords with 1/2 and sometimes even 1 inch cotton rope. It's not as flexible as the retractable leashes and there's much less chance of the dog becoming tangled. We've never had one of those break even when a 65 pound dog hit the end at an all out run. With the rope check cords--wearing gloves is highly recommended!

Wouldn't recommend it for the smaller breeds though!