The amount of leeway depends on the thread overlap in the spoke and nipple, and on the type or rim.
You need the spoke to engage the head of the nipple by at least 2mm. That leaves another 2mm or so to the top. When building double wall rims you can let the spoke extend above the top of the nipple with no issues. However, the amount that nipples thread down on spokes varies with the nipple/spoke combination. Some run out of thread and jam within 1mm of the top, others allow overlap of 5m or more.
Also note that when building single wall rims, the tube will be resting on the tops of the nipples, so you can't have spokes protruding.
So the amount of leeway varies from a low of 2mm or so, up to 7mm or so under some conditions.
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