Only one category ("married children of US citizens"), only for Mexicans, comes close enough to ~30 years total time (~24 years waiting to become LPR, +5 years being an LPR in USA) to consider the "30 years" as valid IMO. The fact still remains that the requirement for citizenship is to be a LPR for 5 years (this is the upper bound, there are cases where less is possible), it is the LPR part that may take time, as I've already mentioned in a reply to my comment. For China and India the total times are closer to 15-20 years. All of this is for family-based immigration, which I'd expect to be generally less interesting as it requires people to already have a fairly close family member who is a US citizen.
