First, Welcome to RCNT.
Second, please turn your caps lock off. Typing in all caps is considered to be yelling in the internet world, and can be very annoying.
Third, to answer your question...that depends on a few things. Which big block 21 are you talking about? The engine will determine some of the answer. How experienced are you in this hobby? If you go by the books and take really good care of the engine, you can extend the life of the engine between rebuilds.
The bottom line is that the length varies and is dependent on a few things. A quality engine that is well taken care of could go between 5-10 gallons between rebuilds. A not so quality engine in the hands of someone who could care less about proper care might only last a gallon or two.
Again, Welcome to RCNT.