every 6 mo. should be ok. You could test with a par meter but they are very expensive
I would go 9 months actually. I always changed them out every 9.
Yep, I go from 9-12 months usually. I find I get more of a green algae that coats the glass when its time to replace.
I get the fuzzy powder like green algae which comes right off with the mag float. You might want to try one of those razor scrapers, I get mine at petco for like $8 and they work great, its a little tedious to do but it will get the job done.