Unfortunately, there's no one easy answer where we can say: "It'll take you four months and 11 days to reach a level of excellent credit."
Excellent credit is a little like snowflakes and fingerprints in that everybody's financial situation is a little different. If you're starting off from ground zero, where you've never had a credit card, you might find that you have an easier time building excellent credit than somebody who has spent the last decade wearing a wig, taking on an assumed name and hiding from debt collectors.
Wherever you're starting off, it could take years before you have excellent credit. That said, don't get discouraged– you can often see improvement with your credit score within months. So if you're trying to establish good financial habits, it may not take long before you go from, say, poor credit to fair credit, or fair credit to good credit, or good credit to excellent credit– but you probably won't go from poor to excellent credit in four or five months.
Lenders like to see a pattern of good behavior, and so three months of on time payments and not carrying revolving debt will make a difference, but it probably won't knock the socks of a lender. But an extended period of excellent financial habits should make a lender sit up and take notice.