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My last new account, related to an inquiry, was in March 2016.
From February 28 to March 2, I saw a nice bump in each of my scores:
Feb 28 TU from 813 --> 826 +13
March 1 EX from 809 --> 815 +6
March 2 EQ from 820 --> 824 +4
Very little else was going on, all card balances were virtually the same, and I had another INQ on TU on February 26 (with no new account associated with it).
So I attribute this to the end of February, ending 12 months since the last new account, and the effect either completely or significantly aging off as of March 1 2017.
After Day 366 (or thereabouts) FICO ignores any given inquiry, even though it stays on your reports for another year. This is true for all FICO models and is 100% certain to be true. FICO's chief competitor is VantageScore, and with VS an inquiry is considered for the full two years.
A lot of people have tried testing to see whether a given inquiry (assuming it caused a penalty) gradually loses its FICO impact as it approach Day 366. I believe they were unable to find any evidence of a lessening impact between Day 1 and 365 (e.g. at month 6). I gather this is also well tested.
Suppose, however, you had exactly one inquiry with each bureau every 50 days staring at Day 1. By the end of the year, you'd have 8 active inquiries with each CRA. I think there is some lack of consensus as to whether you'd experience an inquiry related score hit each time. Some people think you would, some thing you would not. There is also some lack of consesus about how FICO handles CC inquiries that all occur in a short time frame like 14 days -- some think they get grouped as one inquiry and others do not.
Contributor JLK knows the most about it as I understand. He's spent a lot of time thinking about it and tracking his own results.
I think as with many credit-related topics, this one is very profile specific.
The thicker and more aged your file, the less inquiries impact it and for a shorter period of time IMO.
When I took on 3 inquiries last summer their impact to my score seemed to be completely gone after 4 months... and that's also including the impact of the "new accounts" which also lowered my score. There are multiple variables at play here (AAoA can change too, but not always) but when after 4 months my score was higher than before my mini "spree" I took that as the inquries weren't negatively impacting it any longer.