There's no one answer to how long a sacrifice should get or when to remove it.
Generally speaking leave them grow until the trunk is getting close to the thickness you want then cut.
As the sacrifice branch gets thicker the scar when you remove it will be bigger. Larger cuts take more years to close over. There's also the risk of rot setting in and it becomes even more difficult to close. I often remove sacrifice branches well before trunk reaches thickness if I am concerned about the size of the cut after removing it. My preference is to have several sacrifice branches spaced apart. Each adds thickness but being spread makes better taper and each scar will be smaller and heal quicker. Don't always have that option though.
Sacrifice branches can get several metres long. Often taller than the tree but there is a risk it will become dominant and the real tree suffers. If the main tree starts to look less vigorous the sacrifice is chopped - either lower than the tree or right off - even if it has not achieved full thickening. Better a skinny live tree than a fat one that's no use any longer.
Here are a few examples from my benches.
Shimpaku juniper
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Shohin sized Seiju elm
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Another shimpaku
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This virt shows the sacrifice branches that will be converted to jin (white) and the main trunk (blue)
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