When should the driveshaft oil be changed - 2005 Suzuki Boulevard C90

I only own and ride drive shaft bikes. First thing I do is check it to see if it is OK or needs a top up. Then maybe every three years I might change it out.
Your bike is an '05 model so you might want to change it out.
If your going to do it at home, make sure you get the right type for your bike. But get more than 1 litre get at least 5 litres.
Remove the fill up screw, and then the drain screw. Drain all the oil out, put the drain screw back in. Fill it back up again, rotate the wheel a few times. Then remove the drain screw and then drain out all the oil.
Now leave the drain screw out, pour new gear oil in while you slowly rotate the wheel. This will remove all the metal swarf that may have accumulated in the diff. Keep pouring and rotating until what comes out is the same that is going in. Now put the drain screw back in and top up the specs, or just below the lowest edge of the fill hole. Replace the fill bolt tighten and then ride and enjoy.
You won't need to touch it for another 5 years.