In my experience, no, that’s not too much. Especially considering that, as you say, you’ve “never done a proper confession where you confessed all your sins.”
When I became Catholic, I wrote down my sins too, to remember them. (A friend later told me that a priest told him not to do this in case of scrupulosity, so I stopped. But at the time I found it helpful. I’m pretty certain I forget some of my sins between confessions now, but I try to rest in my trust that God forgives even the sins I forget, so long as I confess everything I remember in the moment.) And even with a lifetime of all known sins that I was confessing… since in Confession we just list our sins, we don’t go off on details about them with the priest, I found the time passed way quicker than I expected. For this adult confession of a lifetime of sins, it was definitely under 5 minutes.
So I wouldn’t worry about “length” when you consider this question. In my experience, confessions seem to get overlong when a penitent tries to make it a “conversation” with the priest, and explain each sin or context in detail, or if the priest decides to provide ‘counsel’ on any specific topic. But if you go in with a straightforward (albeit heartfelt) list of the technical terms under which your sins fall, I imagine that will fly by much quicker than you think it will. Certainly fast enough that no one outside will be tapping their foot wondering what’s taking so long.
(Remember, the key to this is that you’re confessing to God, who already knows all the details; the priest doesn’t need to know any details (except if he asks for a detail to clarify what you’re saying); all you have to do is tell him which sins, by the formal name of the sins, you’ve committed, and approximately how many times. Especially if it’s written down so you don’t even have to pause trying to remember, that’ll fly by.)
