In my circle of family and friends, it seems that brides-to-be always ask their wedding party soon after they get engaged. It's normal where I'm from, and it's weird if they don't ask their party right away, and most people will assume that if they haven't been asked a month after the couple gets engaged, that they are not a bridesmaid or groomsman.
So I followed suit, and asked my wedding party shortly after I got engaged. I have no regrets about 3 / 4 bridesmaids, but I really wish I had thought it a little longer before asking the 4th girl. It's not that she isn't "helping" as I don't need or want my bridesmaids to do anything, but she has been destructive. She is family and I added her trying to be nice and build a relationship, but boy was I wrong.
She demanded her parents be invited to my small wedding, she has sarcastically told me she's planning on getting pregnant on my wedding day (like I would care), called me cheap, and talked about me behind my back. I really try to avoid drama, so I don't confront her - I just ignore her and go on my merry way
I just wish I had waited. If I waited even a few more months before asking my maids, I would have seen her destructive behaviour and changed my mind.
Don't make the same mistake as me.
Wait until 6 months out to ask them.