It depends on the type of rocket you want to make!
For NASA they can build a rocket quite quickly if it isn’t got to take people on it and even quicker if they don’t have to transport a lot of stuff! The more stuff you have the heavy the rocket is and the bigger the engines you need therefore the longer it takes to build.
For smaller rocks that we don’t use to go into space, they can take less time, say less than a year to make, because they don’t need as much power because they aren’t breaking through the atmosphere.
Fireworks also count as rocks and because they are produced in large amounts they probably only take a few minutes to make!
There is a really cool rocket that you can make, all you need is a big plastic bottle, a pump and some water! You put the water under lots of pressure in the plastic bottle by pumping air into it until the pressure is so high that the rocket shoots off!
Here’s how you can do it! -http://www.bbc.co.uk/bang/handson/waterbottlerockets.shtml