If the device is reliable then you should quote a FIT number. A very good VCSEL based transceiver in an indoor environment has a FIT of about 100 at Tj~65C and a CI of 60%. If we assume your FIT rate is similar (it won’t be because your operating conditions are more difficult) and have 128 of these devices your FIT rate ~12800 (assuming independent failures). This puts your MTBF at around 8.9years.

Some transceivers have a FIT rate of ~300FIT so if that’s the case your rel will only be 3 years.