How would you translate:
A millennium of days
A millennium in days (Something "in" terms of another thing, for instance, "Thirty feet in meters")

Also жду не дождусь. What are the exact connotations to the phrase and can you say Ждал не дождался тебя

Also also, how does the phrase дверь за дверью work. Could one say тысяча за тысячой лет, or even Буду... тысячу за тысячью лет. (Лет or года, I don't know which). Would mean something along the lines of "for a thousand after a thousand years", which doesn't quite work perfectly in English but does it work better in Russian?

How informal is тыщa (for тысячa), could you imagine hearing Ждал тыщу за тыщей лет seriously in a song, or is that abusrd?