Sure I've got first hand experience. Lifelong sufferer, but only diagnosed at 24.
I've had good use of both Ritalin (both as such and Concerta) as well as amphetamines. I'm currently on the latter, because I seems I've got a relatively high tolerance for the former, which meant that the dosages I was taking was raising my blood pressure too much. (not to dangerous levels, but enough that I could feel it)
I really can't tell much of a difference when I've taken the drugs. Really, no mood-altering effect at all. And for the gazillionth time, it's nowhere near the levels which would cause addiction. (and research doesn't substantiate that fear either) What I use in a month or two wouldn't last a speed-freak a weekend. I have never ever felt any withdrawal of any kind. Going a day without coffee is much much worse - at least I notice it!
The difference, being with concentration and so on, is instead rather subtle. Basically I mainly notice the difference in that days when I've taken the drug are days when I un-coincidentally get a lot of work done. It's not entirely as good an effect as I once hoped for, but it's still a lot better than the alternative.
I think it's awful that the subject has been highjacked by pseudoscience, alternative-medicine snake-oil peddlers, and well-meaning but misguided parent-activists. I mean, hello? There are lots of us adults with ADD/ADHD out there and we're completely capable of speaking for ourselves!
Finally, to express a somewhat radical position myself: I don't believe over-prescription or over-diagnosis is a problem compared to the alternative. Because there's no evidence these drugs are harmful at the levels given, and I simply don't believe people will continue to take a drug if they aren't feeling they get a benefit from it.
I view the 'false negative' scenario as far worse: A kid growing up and being constantly told by parents, teachers and others that he's 'stupid', 'lazy' or 'undiciplined' for behavior he or she is simply not capable of controlling. That's simply child abuse. (And parents are victims in that scenario as well, being constantly told by others that their 'failures' as a parent are to blame)
Anyway.. To the OP: Difficult question. Lack of concentration can be caused by depression, but ADHD is also known to cause depressions. (and has in me, repeatedly) And various antidepressants interact with ADHD medication differently. (SSRIs vs Bupropion for instance) If you're not already I'd suggest talking to a neuropsychiatrist with ADD/ADHD as a speciality.
I agree with RonL here, I couldn't say 'when symptoms appear'. Other than that some times are worse than others, some situations are worse than others and naturally, some tasks require more concentration than others. I'd describe it similarily. We're all constantly barraged with a load of impulses compelling us to do something other than what we're doing.. "Check your email", "Look out the window", "Come to think of it..". ** The difference is, normal folks can choose to ignore these impulses. But in my brain, they're 'louder'. I just follow them (without thought or consideration of course, that's what makes it an impulse') For instance where I left the two asterisks (**) I actually got up and took a few steps around the room, and got back in my chair and continued writing. I can't say why I did that. Just a spontaneous restless impulse.
So my train of thought gets derailed very easily.. But just as the 'derailment' is involuntary, I can't often get back on the right track through conscious effort either. If I can't concentrate, I just can't.
Through good fortune (and/or genes) I managed in school, thanks to good intelligence and a very good memory. (i.e. didn't pay attention much in class but remembered more of the parts I did pay attention to) But you still get stuck with some social issues. It's really hard for some people to understand/accept, for instance, that if I'm not listening to you, it has nothing to do with what you're talking about. If I can't pay attention, that's just it. But people instinctively assume they're boring you, which sucks.
(Making matters worse; the hyperactivity typically makes you an avid talker. What a great social combination.. Big talker, bad listener.)
That's in a way the real curse of ADD/ADHD. It's a 'hidden disorder'. Unlike say, OCD. I don't do anything 'abnormal', like switching the lights on five times in a row. People see that and they recognize that it's involuntary. But those of us with ADD/ADHD.. We don't do anything 'normal' people don't do. Ordinary folks can talk a lot, be bad listeners, get restless, etc.. But we do these things for different reasons. My behavior constantly gets misinterpreted.
To me, (since I can handle the learning and such) that's really the worst, or one of the worst aspects. And not one you hear about a lot, actually.