I've always been told once you see the hoses starting to turn brown from aged rubber it is a good time to replace them - Mine are about 7-8 years old and I may do them again proactively, along with the tank, and maybe the water pump and pulley (will go with EMP Stewart pump and OEM pulley)

If you want replace them proactively, I would say 100k miles would be good time because I do see horror stories on hose bursting on e46fanatics.
Having said that, at 100k, if you don't have previous maintenance records, I would do whole cooling system refresh including water pump, thermostat, expansion tank, hoses.

The 2006 X5 M54 had a partial cooling overhaul in 2015 @ 116K miles.

It is now at 142,500 miles.
The 2 radiator hoses are still fine.
But I may replace them soon.

BMW hose: ~ $85.
Rein Hose (made in Germany) ~ $26.

I replaced upper and lower hoses shortly after I bought the car at around 200k km and at least one of them were Rein and still good many years later at 500k km. That being said the originals didn't look bad either and I actually kept them thinking that if the Rein hose(s) didn't work out I would put them back on!