Lots of factors - I remember very well my instructor and I having a chat regarding first solo. He sort of inferred he might be worried about someone taking 30+ hours. At the time I had about that. So naturally enough I posed the question 'should I give this up'. To which he looked shocked and said, 'no not you, you would have gone months ago if the weather had been OK'.
So I would take from that the first solo timing has a lot of luck involved. If you have done 3 good 'uns, the vis and wind are favourable, chances are you'll be up and off to do the first solo. If that doesn't happen, it may be a lot of hours later if you generally choose to just fly every lesson you have booked, rather than wait for perfect days.
Weigh it up against minimal hours, low cost vs experience of different weather conditions and challenges. I easily doubled the total time to my PPL by preferring to just fly whatever the weather if possible. I was having a great time and doing it because it was fun.
When I did go solo I had already become a bit sick of the circuit and done plenty of dual nav. The actual solo was almost a total non-event, though of course I remember every second of it.
Glad to say these days I see doing a few circuits as no big deal. At the time I hated them because of the self imposed pressure to get that first 'solo' done.