There is no limit on how long a shelf can be.
However, the maximum length of an aluminum bracket is 8 feet. This is plenty to install a 9 foot or longer shelf (with 6 inches overhang on either side). If you want to go longer, you may want to install 2 brackets side by side. Although do note that it could be quite challenging to line up the 2 brackets 100% in line with each other. Using a laser level will help a lot.
Remember that in this case the female brackets in the shelf have to line up as well.
The same is true for the steel bracket, but the maximum length of the steel bracket is 48 inches. And since you are most likely to be using lag bolts (or something similar) to attach the steel male bracket to the wall, lining them up may become difficult.
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