Our first cruise was 7 days, and we felt it was just right. We sailed to the Mexican Riviera, and had three port stops, so there was a nice even split of port days to sea days. Once you know what you like you can select port-intensive vs. sea day-intensive itineraries in the future. Our biggest fear was that DH would get seasick... and he did. But he took Bonine and that resolved it. Now we just know we have to treat for that for him. But it didn't ruin the experience. We ended up loving cruising, and did a 12 day for our Honeymoon, and a couple other 7 days. Now we're doing our shortest - a 5 day on Disney, and I know it won't be enough (but we're limited on vacation time this year).
You're mostly going to find people who love cruising on this board, so the answers are going to skew that way. But when I've talked to people IRL about cruising most people who have done one enjoyed it. There have been a few that didn't -- but I've found that most of the time they went on a line that didn't fit with their style (in fact, many of them went on Carnival as their first cruise, and now think all cruising is akin to tacky, obnoxious, low quality booze cruising). I think
DCL is a pretty safe bet though, if you love Disney anyway.
The majority of the people I know who "hate" cruising are people who have never been on one, oddly enough! They all seem to have this idea in their head that they're going to feel "trapped on a boat" and it's going to be crowded and boring and not very relaxing. But what my husband and I loved about cruising is that he and I have very different vacation styles -- he's a lay by the pool and do nothing for a week kind of guy, and I'm a do absolutely everything because I don't want to miss a thing kind of gal. Cruising is perfect for us because it lets us both vacation in our styles, but together. We can easily split off while he naps by the pool and I go attend some activity, and then we meeet up later for meals or activities we enjoy togethre like bingo or the spa or whatever. We've also never felt like the ship was truly crowded. They are like floating cities and there are so many things to do at any given time that people will spread out. It's FAR less crowded than
Disneyland (haven't been to WDW yet, but I imagine that's way more crowded too). Heck, far less crowded than a touristy area in a major city on the weekend.
A cruise is really what you make of it. Don't feel like you have to do everything if you don't want to. But there are so many different options that I honestly feel like anyone who feels "bored" on a cruise ship is either not even trying, or a really boring person themselves.