When does free diving become to dangerous to do it alone.

Basically I wonder when freediving should be done with a dedicated buddy. I sometimes dive in a community pool with a life guard on duty, unfortunately the pool is only 4m deep. I can also comfortably dive 25m long.

We also have a lake with max depth 17m. I wont go so deep but I enjoy to go approx 5-6m once in a while. I always surface before I have the urge to breath. I am not completely alone, but no one goes down with me.

At what depth should I stop and get a dedicated dive buddy who knows the rescue protocol?