I had a similar experience. When I first started trimix (11 years ago at age 62) 3 ticks on an insulin syringe (0.03 ml) kept me hard for several hours. Now (age 73) it takes 8 ticks (0.08 ml) to get the same effect.
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Obviously, several hours is more time than needed, but the longer time has advantages. At those doses, my erection is ready to use sooner, important with a spur of the moment decision. At a lower dose I may need 20 to 30 minutes to get a usable erection.
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The longer-lasting erection is also good if my wife doesn't come to bed as soon as I expect her to, which sometimes happens. Even if she takes an hour more longer I expect, I'm still good to go.
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Another good thing about the higher longer-lasting dose is that is that my erection feels much more rigid. I love that feeling. It really turns me on makes the climax much more intense. I also like staying hard after sex. It feels good and I find it very satisfying and relaxing to fall asleep while still hard.