Gravity is a good thing to check. If you look up your yeast, you can find out it's 'typical' attenuation, meaning how much it can convert before it stops. My rule of thumb is about 75%
Of course having a starting gravity at this point is good also. If you don't have that, you can get your 'theroretical' gravity by putting your start ingredients into some of the internet beer calculators.
So basically if my SG was 1.040 and my current gravity is 1.020 then I figure I have more ferment (20/40 =50%) but if it is about 1.012 or 1.008 then I figure I'm done (because 10/40 is 25% or 100-25=75% consumed by the yeast). That 75% is a rough value, and verys a lot from yeast to yeast. I think Wyeast lists varrious strains from as low as 60% to others as high as 80% attenuation. I use a SG of1.040 because most starter kits for first time brewers have a target gravity around that.
Now as to the sour, if your at your true end, then it is either 1) do you like the flavor, or 2) is it really too sour? Sour is caused by acid and as a last resort you could try what wine makers do to modify that, but that is a 'hmm I'll give this a try before I pour it down the drain' type resort. Like everyone else here I recommend more time and a hydrometer.