Yeah, flavor extract are for bottling time. Do not boil, or add to the fermentor, they can get pretty nasty that way.
How much to use?
Well, since you're going to add it before bottling, now is the time for a taste test. For best results, you need 2 people and some patience as well as a dose of common sense.
Siphon some beer out of your fermentor, like a pint or 2. Put 2 oz in a tasting glass and both of you taste it. Does it need the extract? I'm serious, many times it doesn't need it or want it.
So you 2 decided it needs the extract, the 2nd person has to sit on the side, or do something else. He/she is needed for the final test only. You add a drop to a 2 oz sample. Use a dropper. Taste a bit. Add another drop and taste again. Keep doing this until you think it is just enough. Try to keep track of the # of drops and keep the tasting sample at around 2 oz.
Now prepare 2 samples for your partner. Mark the glass #1 and #2.
Sample #1: add 1/4 of the # drops to 2 oz of beer.
Sample #2: add 1/2 of the # drops to 2 oz of beer.
Let him/her taste sample one and write down the verdict. Repeat for #2.
Discuss which one is better. But use only 2/3 - 3/4 of that amount.
Now add a proportional amount of your extract (count drops or ml, there are roughly 40 drops in a ml) to your cooled down priming sugar solution. Add that to your bottling bucket and stir very gently, so you're not oxidating your beer.
Let us know how it went.