Build time

Build time refers to the minimum time it takes build a unit or structure assuming sufficient credits are available.

In most Command & Conquer games credits (or money) are the critical limiting factor to unit production and are thus considered the main resource.

The opposite in true in Tiberian Twilight where units cost no credits at all but has significantly longer build times and limited construction rate making time the critical resource.

Red Alert
According to the rules.ini it takes 0.8 minutes to build something that costs 1000 credits.

In actuality when you start building something, the translucent clock image animates to show how long until completion. That image (clock.shp) consists of 55 frames (the first frame happens instantly so its 54 frames technically) and the game never skips those frames meaning that the minimum build speed is always 54 frames of game time and the time to build something is always a multiple of 54. The original Red Alert had a few rounding errors, while remastered fixed some of them meaning the build speeds might differ slightly between games. A good example is that 2 barracks are needed to build Rocket Soldiers at the maximum speed, while in remastered it takes 3 barracks.

INT(Cost * 204 + 128) / 256) * 230 + 128) / 256) / Factory / 54) = Frames per clock image update. Spreadsheet

where Factory is the amount of Barracks, War Factories, Naval Yards, etc. the player has (since its integers only, it rounds down). This formula was taken directly from the executable file. Aftermath limits the Factory bonus to 2.

Emperor: Battle for Dune
Every 32 units of build time equals about 1 second of real-time with the game speed set at 4.

Formulas
Tiberium Wars and Red Alert 3 have set build time for each unit and structure that does not rely on factors other than power usage and is visible in-game.