If you're using a Bosch WB, then within a couple feet of the Turbo is fine, too close with it, and you can *overheat* it, Innovative at one time sold a deal for helping to shed some heat if it was too close.
The NTK (Honda) can be about anywhere. It's much harder to *overheat* since it has a 850dC operating temp..
The other item is that if you engine is passing much in the way of particulate matter in the exhaust then too close, can cause them to actually stick to the sensor, and that will kill it. To avoid that means mounting it further away, ie closer to say where the cat would normally be. FWIW, I've had a turbo oil seal failure, a piston failure, a set of headgaskets let loose, and the sensor has survived.
You want to mount it, so that the sensor is tilted above horizontial, so that condensation doesn't accumlate in it. Mounting it in between the 10:00 and 2:00 position helps, thou you still need to have it so that it's pointed down, and not tangent to the pipe.
But, I'm no super tuner.