How can you tell when empty?

Sounds like maybe something didn't work right. The "window" at the back of the bottle should've shown the level going down and that is a large space to cover without it moving at all.

We apologize for the problem you experienced with our product. We would be more than happy to assist you with this matter. Please contact our Consumer Affairs Department at 1-877-229-3724.

Since your product is a year old the valve might be clogged with calcified product. I had a similar problem with a similar product, I sprayed for about 20 minutes and realized the amount in the container didn't drop any at all... I messed with the valve and found it to be faulty; I replaced it with the valve from a like product and resprayed the area the following day. The liquid dropped considerably with that application. You know it's empty when you no longer can see the product via the skinny view area down the side of the bottle. The water does not enter to bottle, the pressure acts as a siphon and mixes the chemical with the water.