I don't know what the etiquette rule is, but based on experience, try to give people at least a month to find out if they can make the wedding. It can often be difficult to plan around work, kids' school, etc.
My wife and I just got married a few weeks ago. In the invitations, we requested that the RSVP's get sent back by about six weeks before our wedding date, but they actually slowly trickled into our mailbox until about two weeks before the wedding. It was kind of annoying and stressful that we were still receiving RSVP's several weeks late, especially when we needed to get final meal counts and table assignments to the caterers ASAP.
Not everyone realizes how inconvenient it is for the Bride & Groom to receive a whole bunch of late wedding RSVP's, or worse yet, not receive some at all (because then you have to dig up a bunch of phone numbers and call everyone, when you have a bazillion other details to finish planning). People will send their RSVP's back late, or not at all. The more time you allow yourselves to figure out your guest list, the easier it will be for you.
If you can send out the invitations at least three months before your wedding, and request that the RSVP's get sent back at least two months before your wedding, you'll have a lot less last-minute stress. I wish we would have sent ours out sooner than we did.
I don't know how big your wedding is (we invited about 160 people and 90 ended up coming), but if I were you, for a May 23 wedding, I'd send out the invites around February 23 and request that the RSVP's be sent back by March 23.
Congratulations!