My opinion is not popular, but you asked!
Our family never wore a button other than on our birthday itself. Not the whole trip: just one single day. The buttons used to look like this:
"TODAY IS MY BIRTHDAY" and that's what we stuck with.
What happens if you're not there on your birthday? My
opinion is that if your birthday is/was within a couple of weeks, go ahead and have fun. It's close enough and you're still in the birthday mood. Get the button and go to town. If it's not, then I simply don't understand the need for an adult to "celebrate" months and months before or after the real date. Yeah, I've heard the arguments before ... "I/DH/DW/DD/DS always work on my birthday." or "This is my birthday present to myself." and "This was the only time it worked for our family." I get it that people want to make the trip special, but for goodness sakes all of us are within 182 days of a birthday. A birthday is really not all that special. Notice that I said "adults". Kids are completely different and it's always OK for them to wear a button.
All that being said, Disney now wants you to "celebrate" your birthday, your half-birthday, your 8/12ths birthday ... whatever. The know that if you are "celebrating" something you are far more likely to open up your wallet for The Mouse to take what he wants. "Let's splurge! It is/was/will be my birthday within the last/next 182 days!!!" So Disney doesn't mind you celebrating. No they don't! In fact, they encourage you to "celebrate" whatever you want, whenever you want.
To sum it up: My family only wear the button on the birthday itself. My opinion is that a button is OK if your birthday is close to when you're there. Disney wants you to "celebrate" and spend money so it's a-okay by them is you grab a button and wear it your entire trip months after your birthday so you should go ahead and do what you want.