Definitions

Reliability is most widely defined as "the ability or capability of a system to perform the specified function in the designated environment for a specified length of time".

As a result the life of a system can not be determined except by running or operating it for a desired time.

Let's look at a real example:
A client of mine who owns a travel business called me this morning, he was disappointed that a new hire in his firm was 2 hours late yesterday and today did not show up. This disappointment is basically my client's feeling that the new hire is not reliable. During the relatively short amount of time the employee joined the firm, he failed to meet expected a capability (showing up on time). These two events dramatically undermined the employee's reliability. If the employee had been late after a year of joining my client's firm instead of a few week, his reliability rating would have been much higher.

Conclusion:
That is why first impressions count most, and should be given consideration.

In engineering terms this is what we call Mean Time to Failure, i.e. the amount of time it takes to fail, from an non-failed state, on average.

Mean Time Between Failures is another important characteristic, it tells us how much time it takes to fail again after the previous failure, on average.

Failure rate indicates how frequently a system fails per unit time, typically identified by the greek symbol "lambda"