If it were me I would bring them to a Bosch shop and have them tested. They can get the replacement nozzles direct from Bosch and rebuild them if necessary. I know you can buy replacement rebuilt injectors from Bosch for around $100 a piece which is also an alternative.
AFAIK there is no "rule of thumb" interval for testing or replacement of injectors. Basically if you have no symptoms of trouble (like smoke, nailing, roughness) I wouldn't go looking for any - but that's me. Mine have 165K on them and have never been out of the engine and all is well. I suppose if I had an extra $600 kicking around I would replace them at 200K but I don't so I won't.