Asking how many hours of time is needed to learn something really is an impossible question to answer. Age, current level of competence, how fast/slow one learns and, overall level of dedication and motivation to learn and talent level are just a fraction of that equation.
The only half decent answer is "as many hours as it takes you to learn the piece!"
In my teens and 20's a song like Czardas was be a "3 days to learn, 2 days to memorize and perfect" piece. I had a 250-300 song repertoire in my head. Today if I had to, starting fresh, a similar piece is a month or more, and forget the memorization part... I have so much info in my head that when I sleep, data seems to fall out, and I seem to wake up each day stupider than before... lol
In the last 2 weeks I've committed to returning to Free Bass (again... lol), and I had set the goal of 5 pieces that I thought would normally take me 2-3 weeks to be able to play... yeah right. In to near the end of week 2 and I am at the point where I am playing the entire FIRST piece (a 7 page song), with a ton of errors... lol
The last 3-4 years time to practice has been a precious and limited commodity for me, and to force myself to take 15 minutes here and 15 minutes there to play is the very most I can find, so I am sure that is part of the issue. The 2nd issue is zero muscle memory and pretty much a relearning from scratch on the left hand, but I am actually pleased as punch that this is happening, even at this snail's pace!
Hi Jerry, I have been reading in a book written by a psychologist, that there are NO such thing that "talent" and "genius", only DILIGENCE and tons of PRACTICE...
Of course, I do not doubt that you may experienced some decline of your abilities in the playing and memorizing, but in such case I am dare to suggest you something: take a try with the "Carnivore diet"!
I also follow that, this is why there are in my nickname the first word "Carnivore".
It is a well-proven fact that the memory and physical skills both get serious improvement when the body switches from the sugar-burning metabolic state to the nutritional ketosis.
How to switch to that? Well, I think this would be here rather offtopic, but there are plenty of videos about this topic on the Youtube, and printed books too. (For example Paul Saladino: The Carnivore Code).
I did it, I am on this diet since the last 2 and half years, and I experienced only benefits (except the first 2 or 3 weeks, the transitional period, naturally). My weight was 91 kg, now only 79. I had itchy skin, aknes, chronic fatigue syndrome, and so deep depression that in multiple times I almost committed suicide. And I was so irritable that on more than one occasion I yelled back at my boss in such an insulting way that I wondered why he didn't fire me immediately. Now, all these bad things are GONE!
So I just suggest you friendly, take a try... I deeply believe, your musical skills will get serious improvement, you will be full with energy again...
I tell you these things based on my own experience! It was me a so useful great "metabolic adventure" that I had been writing even a book about this diet, of course in my native tongue, Hungarian, so I do not link it here, supposing that there are very few Hungarians in this forum, if any at all. And anyhow, that book is a sci-fi, not a detailed dietary curse. But seriously, take a try! if you have memory loss, or just less "brain resources" in general, then surely, it is a perfect sign that your brain cells have some nutritional deficiency.