I usually see 3 to 5 years out of these heads depending on use and how well the machine is maintained. I have seen some fail earlier due to high use(3 shifts a day 7 days a week which the machine is not designed for), poor maintenance(most common reason for failure) and/or bad head strikes. If you change your dampers and cap top at least once a year you can avoid a lot of problems.
The Mimaki extended warranty costs about as much as a new print head, maybe a little less, so you run the risk of not needing one and not getting the full value out of your warranty. On the other hand, all of the other parts are covered under the warranty and it may be worth the piece of mind. So if you don't get the warranty and need a head later on you will be paying about the same as you would if you just paid for the warranty but you may run into other problems that end up costing more in the long run.
If I were running a shop with a JV33 and didn't get the warranty I would have a separate savings account with about $3500 in it as a form of self insurance for yearly emergencies. $3500 is enough to get a print head if needed and will cover most other parts on top of a head. If you don't end up using it, it makes a great way to have a down payment on a new machine when you want to upgrade.
