I actually have a client who wanted to test the limits of CRM NTEXT fields this week. Typically at about 5,000 characters most people think that's enough. However, this client had the need for more. All documentation from Microsoft states that 5,000 characters is the limit but my client wanted to see what would happen if he went over it. So, he attempted 100,000 and the system took it. We then tested putting in 100,000 characters of text and the field took them all and saved them to the database. We didn't feel like we needed to go higher than 100,000 characters, so I haven't found the "actual" limit of NTEXT fields, just know that they will take a lot.
Now, the downside to doing this is that your database now has to store a column that can handle 100,000 characters so it eats up the space in that table really fast. So, I don't recommend doing this on more than one field per CRM entity. Not that people really do duplicate checking on NTEXT fields, but there would be no way for you to duplicate check against this large NTEXT field. But since most NTEXT fields start at 1,000 characters you can't use out of the box duplicate checker on those fields anyway.
Anyway the point is that Microsoft built something that can handle some really big stuff, it's not recommended but still, pretty sweet if you really need it.
Dynamic Methods Inc.