How often do i clean filter?

What is the recommended interval for cleaning out the filter
(rinse with dechloribated or used aquarium water of course)
for a;

33gal tank
Aquaclear 300
4 goldfish about 1-2" each?

I siphoned the gravel 4 times this week, did a 30% water change each time,
my nitrates wont go down, they are very high 110mg/L. Do high levels
cause fish to swim radically? Fish seem normal but when i walk by the tank
The bigger one especially seems to go nuts, he can dart from one side of the
tank to the other
so fast he smashes into the glass basically goes nuts for a few second and
then calms down.
Is this a sign of Nitrate poisoning? How bad is it? Nitrate by itself I
thought was generally harmless.

I cleaned the filter media tonight, I usually do it once a month, is this
too long an interval?

Thank you and best regards... Kodiak