Dosage - How long is a 2.5 ml bottle supposed to last when you use 1 drop in each eye per day?

The number of drops per ml of any medication varies depending on the viscosity and density of the solution. Also some manufacturers overfill the bottles to provide extra doses. The prescriber info for Latanoprost solution by Sandoz states that each bottle of 2.5ml contains 24 drops per ml, which is 60 drops The brand name Xalatan by Pfizer is overfilled and contains more than 100 drops per 2.5ml bottle. Studies have been done to analyze the drops per ml, quantity per bottle and cost of use for all manufacturers of latanoprost. Patients will argue that they receive much lower than 20-24 drops per ml. However, a general rule of thumb is 20 drops per ml unless specific info is provided by the manufacturer.

25 days.

I think it's 2.5 ml in total solution 4 ml, thus 80 drops.

There are 20 drops per milliliter. If you have the 2.5ml bottle, that's 50 drops. If you're only using 2 drops per day, it will be a 25 day supply. I'm a CPhT, so I know what I'm talking about.

1ml=16drops so 2.5ml is about 37 drops so a 2.5ml will only last about 2 weeks and 3 days.

More or less 2.5ml of eyedrops, with the usage details you are giving, should last you 15 days.