I'll reply publicly, as others may be wondering as well.
Response time depends greatly on the volume of requests that the historian must address. My impression is that the lag is not great these days, with some people reporting recently that they have received a letter in about three weeks. If you figure a month, plus or minus a week, you are probably in the ball park.
There have been occasions when the response time was two to three months because of the backlog of letter requests.
The queried model should not have much to do with wait time. That's three to five weeks for almost any gun the company manufactured.