How long can I reuse this delicious bacon grease for cooking?
Is it safe?  maybe.  Is it recommended?  No.

If you're going to save bacon grease (or other fats), I'd recommend:


straining it
putting it into a different container to minimize the surface area exposed to air
keeping the container in the fridge.


Although, the first one, I admit I don't do -- as I'm not cooking bacon every day, I've generally used up my supply by the time I've got more ... so I fill a glass jar, and let solids settle to the bottom ... then just use the stuff off the top as I need it, until I get so far down the jar that I hit the darker strata, when I then dispose of the whole thing.  (I recycle jars for this, then pitch the whole thing ... you don't want to go washing bacon grease down the drain, it does nasty things to your pipes)

...

If you're going to do what you are, you'll want to make sure that you hold the pan at 250°F (121°C) for at least three minutes, preferably longer, to kill off botulism spores, unless you're fond of that whole paralysis thing.