A good automotive shop or engine rebuilding shop can rebuild the engine within a week depending on how much work needs done such as machining etc.
But it can take 2 to 4 months to actually get the engine rebuilt by an engine rebuild shop depending on how busy the shop is.
Expect to have the shop take at minimum 2 months but as long as 4 months to even a year in some cases to have the engine returned to you.
Sometimes you can just buy a rebuilt engine from the machine shop that is ready to install in your vehicle and then turn your old engine in for a core charge.
The machine or engine rebuild shop can then rebuild that engine and sell it to someone else or you could also have them rebuild your current engine and return it to you after you pay for the rebuild and keep the motor for a spare engine.