Like you, we have a 1604 and 1624 here and they are usually pretty easy to keep them up, even when things slow down and you're not printing all day, every day. I would advise running them as often as you can. It might be a good idea to run some promotional items, small banners, etc. for your shop? Or perhaps donate a few banners to local churches, or non-profits that will both help you and them…?
Regardless, the key is that the cap top is in good condition making a tight seal with the head, and also that the drain lines are clear and free from any clogs or blockage. I have no fear leaving mine turned off for 3-4 days at a time, again condition of cap top and knowing your drain lines are clean is key. When you fire them up do a quick wash, print a little something to keep the ink moving in the lines and shut them back down. Eco solvent ink is very forgiving and even if you do get a few nozzles plugged after sitting a while, it is usually an easy remedy if the rest of your machine is in good working order.