I tried cherry extract for the first time a couple weeks ago. It was a 4oz bottle and the instructions said to use the entire bottle for 5 gallons. I didn't trust this so I poured 1 cup of the beer into a measuring glass and started adding drops of extract in with a small pipette. I put 10 drops into the 1 cup of beer, stirred, and it tasted like nasty cough syrup. I decided to use half that amount for the beer I was about to bottle, so I did the math and actually counted out 438 drops that I ended up putting in the beer. I measured the cherry extract prior to dumping in the beer so I would know how much to use next time, and it turned out to be 2tsp for a 5g batch. This does not give a strong cherry flavor - it's subtle, but it does not taste like cough syrup.
FYI - 2 tsp barely makes a dent in a 4oz bottle - I could probably use the entire bottle for 20 batches.