I raced dirtbikes for a couple years while off the road and we'd find dirty fork oil after only about 10 rides and change it. Of course, streeters don't have the travel distance, speed and general abuse the mudslingers do, but if you have a drain plug and can get a fingertip of oil out to check it's color- and look at it in sunlight for metal particles - I'd change it if I saw anything dirty.
Oil is cheaper than parts. Dat's da law, man.

 

In the instructions for the FX SOFTAIL FRONT LOWERING SUSPENSION KIt 54621-05, step 18 states,

Adjust the fork oil level, so that it is 5.55 inches (141 mm)
from the top of the fork tube with the fork tube compressed
and the main spring removed. (they have a special tool that will enable
you to suck out the excess, something like a Mityvac.

That would indicate that there is a method for adjusting the level of the fork oil and that method is to adjust it by sucking oil from the top end.

I have not tried it but it might be possible.

I just replaced my lowers with chrome lowers and when I drained the oil it really smelled, it stunk up the whole garage. I think that I will try to see if I can ensure that the oil levels are equal by sticking in a piece of ridge brake line as a dip stick. Let you know what I find out.

 

Here's a link from the Dyna section that may help you with this. I've just rebuilt my front forks this past weekend. You'll definitely want a a shop manual IMO. You can check the level between the shocks by removing the caps and comparing the levels. It not so important that you get the recommended OZ of fluid in the forks, its more important that the fluid levels match each other.