Inv_AeX_EU_Logon_Users runs every day I think (I'm not sure; we have completely hacked up the Out-of-box config for Inventory).
yusgab's report will find any machine where the primary user hasn't logged on in > 90 days, but I haven't analyzed it enough to know if it will tell you if ANY user hasn't logged on in that long. It might also be worth adding the Inv_AeX_AC_Identification.[Client Date] column, which has the current date/time on the system when it last sent basic inventory (which should be every day). I think this will work:
Select vc.[name], vc.[IsManaged], MAX(eulu.[Last Logon]), acid.[Client Date]
from vComputer vc
JOIN inv_aex_ac_identification acid
on vc.Guid = acid._ResourceGUid
JOIN Inv_AeX_eu_logon_users eulu
on vc.guid = eulu._resourceguid
WHERE eulu.[User] IS NOT NULL
AND eulu.[User] <> ''
AND DATEDIFF(DD, acid.[Client Date], GETDATE()) < 7
AND DATEDIFF(DD, eulu.[Last Logon], GETDATE()) > 90
GROUP BY vc.Name, vc.IsManaged, acid.[Client Date]
ORDER By vc.Name--, max(eulu.[Last Logon]) DESC