The sentence refers to a future point in time (January 1999) and an action or duration that will have been completed by then—this calls for the future perfect tense. "Should have been" This is a form of future perfect. It expresses that by a certain time in the future, something will have happened. 

 "By January 1999, I should have been ten years in the service..." (meaning: by that time, I will have completed 10 years.)

"would be is conditional/future in the past, but it does not express completion by a future time. "can be" is about ability, and doesn’t suit the context. "must be" implies necessity or strong certainty, and doesn’t convey the idea of completed time.