liquid test kits - expiration?

I looked around and I didn't see a good answer on this. What is the shelf life of liquid test kits?

I purchased an API master kit about a year ago. The nitrate tests are about empty (since they take 10 drops each test), but some bottles still have quite a bit left. pH for example - I rarely or never test for that.

Is there a point that liquid tests need to be thrown out? How long are they good.

I don't think there is an expiration date on the bottle and who knows how long they may have been sitting in the store. Any key to the lot number on the bottle?

Maybe there was a bar code on the original box - but I don't see a barcode.

There is a lot number on each bottle that looks like "17A1006". The first 3 numbers/letters are different on each bottle. Most of them are either 1006 or 0906 on the last 4 digits. Is that a manufacture date for the lot???

Thanks - I didn't pick this up when I searched.

Just curious - where did you find the durations for the different tests? I looked on the API website but didn't see anything.