How many days should I check for FG before knowing fermentation is done?
Is there a way to know estimated FG based on OG?
check first when things settle down, krausen drops. check again in ~ 3days. if it has not changed at all it's done. you can leave it alone for a while longer if you wish/must. but mostly let taste be your guide. how does it taste on that second sample day? if it's clean and tasty and the gravity hasn't changed you are good to go at any time.
you can also perform a forced (or fast) fermentation test. Take a sample, say 10 floz, of your wort into a quart mason jar and pitch a LOT of yeast into it. preferably the same strain you are fermenting with. Keep that sample somewhere warm (not a problem in FL this time of year i suspect) and give it a swirl every once in a while. after just a couple days that sample should be obviously done with yeast dropping out and everything. take a gravity reading on that. that is your theoretical maximum attenuation for that wort.
you can't really estimate well based on OG because every wort has a slightly different sugar profile and will ferment to a slightly different FG. with extract brews it's easier to estimate if you stick with the same maltsters products because they are pretty careful to produce consistent product.