I just wrote them out by hand with a nice pen. Took me like 2-3 evenings with H's help. I probably could have done it in one, but my hand got tired. I had about 100 I sent out.
You're within 4 months, so I would definitely look to start ordering. You'll want to mail them around 6-8 weeks out.
It all depends on how you plan to do it. We did a kit from Party City, I spent one night printing the invites out. With creating the wording, finding the right font & then printing 100 invites out I think I spent about 2 hours on that. Then the next night I did the RSVP cards, again I think between designing them (with help of online template) I spent around 1-2 hours doing those. The envelopes took me longer. I was able to print right onto the envelopes but I had to type in each address new for each envelope, but that allowed me to pick a nice calligraphy font and it looked so nice. That probably took me 3-4 hours total. Since I figured out how to print on the envelopes I also printed on the RSVP envelopes our address plus the guests address for the return address. This came in handy because we had a few guests with similar names and one guest who forgot to put their name on the RSVP card. But I figured if something happened and the RSVP card didn't get delivered to us & was "return to sender" the guest would get it & know we didn't get their RSVP. Again that probably took me about 3 hours or so. So I probably went a little above and beyond since I was able to print on the envelope but for me it was easier and nicer looking then writing them out because I don't have the best handwriting. Putting the stamps on everything maybe took me 15 minutes. Assembly was probably 30-45 minutes. I only had invite, rsvp card, rsvp envelop and a business card that had our website address on it for directions, hotel, etc info.The business cards took me probably 30 minutes to make
For me, doing one step per day, made it less stressful.
It took (I think) about two days to assemble the invitations (front/back/cardstock) and stuff the envelopes, but it was two of my days off, so it was probably around 10 hours or so. Addressing was the fast part - it only took a couple of hours, because I used my normal, nice handwriting and just motored through. We also had a rubber stamp for the back of the envelope, so it took probably about an hour to stamp them (in batches) and let the ink dry so it wouldn't smudge.
I'm the type of person who starts a project and just goes straight through until it's done, though, so doing a one step a day would have made me wonder when it was ever going to get done.
And the suggestion to just address 12 an evening? Lawdy lawdy, it would have taken me two weeks just to address them. Good grief. Once you're in a groove with something, just keep going