Hi Paul,
You generally have it right. The short answer is after 90 seconds, an SIA-Query is sent.
The longer answer:
Each router manages its own SIA Query and SIA Query Response management. This means that as soon as each router has put a route in Active (regardless of whether the router in question was the actual first router to start the process) and has sent a Query to its neighbors, that router also starts a timer. At 90 seconds, for each neighbor from which it has not received a reply, it will send an SIA Query to check on the status of the original query. Any of those other routers that, themselves, are also waiting on a Reply from others will respond back to the router that gave the SIA-Query with an SIA-Reply. Any neighbor that does not respond to an SIA-Query by the time the original timer reaches 3 minutes will be considered stuck in active.