PondFlitration Filter Questions

I have a PL 4800 with the 3 'media' bags and UV light. We have not done
anything with this filter since we inherited it (we moved into this house
with the waterfall/pond already here). Last summer was our first summer with
the pond, we rinsed the media bags and put them back in. The pond was
reasonably clear, you could see the bottom but it was a little green and
hazy. Thought that was normal. Did the same with the filter this year, was
the same green, hazy clearness for a little while than the pond mucked up in
part due to a leak with the waterfall. We had to shut down the pump to fix
the waterfall so I bought a PondMaster filter/pump to keep the water moving
while the waterfall was off. To my amazement the water was extremely clear
and NOT green or hazy after a few hours of the new pump/filter operations.
Long story short, waterfall fixed, no leak back running thru' the pl 4800
water is green, hazy, murky can't even see the bottom. SO, now that leads me
to my questions.
How long do the media bags last? We are fairly sure these are the original
bags, we ran the filter last season for 4 months and this season for 2.
During the winter we shut the waterfall down and will be using the
pondmaster filter. Obviously, the media bags need to be changed out just
need to know how long they last with summer use only.
Once we have replaced the media bags how long before the water will clear
up?
Also our filter has a UV light, we don't even use it the bulb is no good. Is
a UV light necessary for the proper filtration of the pond?OR vital to the
filters operation? Do they really work? I have read some conflicting
reports. I know it is for algae but we used barley straw last year and
didn't have any string algae just the little growth on the rocks which the
fish eat off of. We used the Barley this year too still no string algae but
do have green water.
Appreciate some input on this, THANKS!