SalinValu

It used to take a while, but they massively sped it up shortly after New War released. The Rising Tide quest is all about building a railjack. To start it, you have to build a thing that takes 10 minutes. Then during the quest you go on a series of defense-style missions to collect blueprints of each of the six parts. Each takes only 1 minute to craft in the foundry.

So, if you have all the necessary resources, it'll only take 16 minutes of active build time. The mission time is the longest part now; I don't remember how long the missions take, but they aren't that long.

Yeah, they are different/random for everyone. The only one that is the same for everyone is T9, since that's always 25x steel essence.