Question: I would like to have an information about the MTBF of the nRF51822 and know if you have any document about the test condition for these values ?
Answer:
Attached is a PQR report for nRF51822, which states the FIT in entry #11.
FIT (60% CL) : 51 (given in 10^9 hours)
MTBF = 1 / FIT = 1/51 * 10^9 hours.