I have yet to plan and book my own cruise. The 2 I've been on and the upcoming one have all been done through a 3rd party, because either 1 or both of my girls were dancing on board. I'm currently mulling over the idea of one in Feb 2025 during my birthday. Once we figure out if it's in the budget, we're going to use a TA that we know, and hopefully can get us a good deal.
I really want to have the control in my hands, though, because for the upcoming cruise, the details were originally to sail with Royal Caribbean and (due to some of the parents not having their money in on time), it had to be changed to Carnival. I'm ok with that, since we're gaining "status" with Carnival, albeit very slowly and not very much each time. Once everything was settled, it took forever and a day to get our booking numbers, but finally two of them trickled in via email. Problem was, there should've been a third as well. So, I bugged my wife to ask the studio owner about it. Over, and over, and over, and over again, to the point where I called the booking agency and got it myself. I don't consider myself to be a squeaky wheel, but I wasn't about to wait until March or something to get our last booking number, because we also decided to book excursions, and I'm not exactly patient sometimes. That has all been worked out.
I think a year to 18 months is plenty of time. It allows you to plan for excursions (if you're so inclined), and to pay it off at a reasonable monthly rate. Whatever you choose, I hope you have a fantastic time!!