Do the flush every 3 years. Inspect the hoses with every service job, and if they don't feel "soft" I wouldn't worry about replacement. Keep an eye out for signs of seepage at the hose ends. If you see signs of the hose seeping (either actual coolant loss or the chalky white/yellow residue) then attempt to tighten the appropriate clamp, and if it is as tight as possible, then replace that hose.
If you do your own service work, i'd recommend buying a few jugs of the Mercedes coolant to use.