I feel like it depends. I think technically it's a year, but I think that comes from a time when people didn't live together until they were married, so experience for the first time together a lot of things that modern couple experience earlier in their relationship. For me, I basically stopped feeling like a newly wed when we got home from our honeymoon and we went back to our normal routine of working. Not that it's a bad thing, it just didn't have a long lasting effect on me.
What an interesting question! Personally I think a newlywed would be anyone married within the past year, but that's just my opinion. I took my honeymoon one year after I got married and I know I still felt like a newlywed then. Now that I've been married for, gosh, more than two and a half years, I think I need to consider myself an old married lady - lol!