How soon does it switch from Administrative Processing to Ready to Issued? Is there a particular pattern we should watch out for as a sign that our visa application was approved?

One, there is no particular pattern.

Two, nobody (even if they claim they can) will be able to tell you when you can expect your Visa Status to change on CEAC. Every case varies and you really don’t have any choice but to wait and check your status repeatedly for updates.

No one should finalize travel plans before having visa is on hand. Don’t assume that your visa will be issued after just a few days after interview. Else, you are just in for a massive hassle (and headache) with re-booking plane tickets. There are a lot of unforeseen circumstances that may affect your visa processing / issuance / delivery namely weather conditions, holidays, additional administrative processing/review, etc.