How long does contact lenses solution last for you?

I've been using bio true contact solution and each bottle is 2 fl oz. I've been wearing trial contacts for less than a month (since May 20th) and have already used up 1 bottle and about 3/4 of another. I follow the directions from my doctor and what the bottle says - 3 drops on each side and then I fill the container with fresh solution every night. The bottle says the solution is safe to use for up to 90 days but should be used up before then with continuous use (it doesn't say when, though).

Am I using too much? The first bottle went a bit faster, about 10 days, because I had a harder time putting in my contacts and would constantly drop them and have to use more solution. How long does a bottle typically last for you?

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