It was thirty years __ he was invited as an outstanding alum

It is…before…是个固定句型.该句型的is/was后可接long或一段时间.肯定句译为“过多久…才…”；否定句译为“没多久…就…”.例如：
It was a long time before I got to sleep again.
It will be hours before he arrives here.
It was not long before he left his home town.
It will be five years before we meet again.
It wasn’t long before he told us about this affair.
It was evening before we reached the little town.
这类句型一般译成“过……才……”或“不久……就……”,需注意的是前后的时态要相互照应.

C 30年后他才作为一位优秀的毕业生，受邀参加校庆28周年纪念日。