An SBR won't take long depending on how you do it. If it's a pre built one the process is almost exactly like buying a suppressor. It could be a year to get an approval.

In this scenario you are getting a form 4 approval.

If you are building one yourself you can put your data in via the website and it takes around 3 weeks. The last time I did one it was right on 3 weeks or so.

In this case you are getting a form 1 approval.

The difference between form 4 is that someone else built the gun or suppressor. (IE factory built).

With a form 1 you are the 'builder' or 'manufacturer'.