How to keep the time before coffee get out of the machine? I just bought a Nuova Simonelli Musica + Eureka Mignon, so I'm practicing extracting a shot.

I am trying to make the extraction lasts 25 sec.

But when I start the extraction, I first have preinfusion time that last like 3-5 sec, and then the pump starting, that is lasting like 3 sec also.

So, at what time should I begin to count 25 sec ??? At the beginning ? After the preinfusion time ? Or when I see expresso out of the machine ?
Together with the first drop

That 25 seconds we curved in our brains is mostly related to the speed of the extraction. Mostly, under normal circumstances, with a single spout portafilter for a single shot espresso, we expect to see 1 ml per second extracted coffee. At the end, you collect 25 ml's of coffee in your cup.

So, I suggest you to count the time after you see the first drop.