Depends. Whats the amperage rating on the bulb and the battery? Is the 12 volt bulb LED? The equation to find your answer depends on how much your 12 volt bulb is drawing and how much your battery has. I have seen 12 volt bulbs that are searchlights and I have seen 12v bulbs that are little LED ones. Huge difference in the amperage draw which will have a massive difference in your answer to how long your 9 volt battery will last. Same goes for the amperage of your battery. I have been in the military and seen 6 volt batteries that are smaller than a AA and I have seen the 6V at Walmart that are decent sized and much bigger than a 9v.
If you want to power your 12v bulb for a long time, connect two 6v batteries in parallel. That will give you lots of amperage and a full 12 volts.
Another Answer
A 12-V incandescent lamp, operating at 9 V, will operate well-below its rated power and will, therefore, last much longer than it it were operated at 12 V. Further to the original answer, lamps don't have 'amperage' (current) ratings and is certainly unrelated to 'how much the battery has'! Finally, in order to achieve 12 V, two 6-V batteries would need to be connected in series, NOT parallel.
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