Hmm, good thread.
Well, here's what I have to say for you. Smash is a wonderful, fantastic series. I'm not a competitive player by definition (I don't use all the advanced techs or enter tourneys), but I believe I'm about the best I can be without using the uh, advanced advanced techs. (Wavedashing, etc)
1. Time you've felt brought you to your current level
I've been a huge fan of Smash since the N64 days, so it's really hard to say. I'm pretty dang good at Smash, but I feel like my skill hasn't come from all the years I've owned it, but rather the last three or so, where me and my friend were bored one day, picked up our copy of Smash we haven't played in months, final destination no items, and fought. We forgot how fun it was, and played battle after battle of Young Link vs. Young Link, our two mains. Eventually this turned into "Random Battles", and we're completely addicted to them to this day. Random battles is random character no repeats, random stage, all items on very high for two minutes. But we started doing this a lot, and I picked up a lot of skill. I haven't played much one player since then until recently. It's funny, all the things I remember having TONS of trouble with (Enough to break a controller from frustration) took me one, maybe two tries this time around.
Long long story short, it doesn't take a massive amount of time to reach your level. My tip? Find a friend to play with, it's the absolute best way. I played here and there a lot for years, but it wasn't until me and my friend started having our constant matches that I got really good. Fighting against computers is good, but you don't really learn effectively unless you're fighting a human. I bet you could get to be at least really good within a year, honestly. Or maybe that's because I just pick up games faster than others. =/
2. If you feel you've invested enough time to reach your level
In a word, yes. Like I said, I truly believe it's only been the last few years where I've truly built my skill, and it's certainly been enough. I still learn something new almost everytime I play. I think personally, I'm good enough to enter a tourney, but I wouldn't really know. I'm either super exaggerating in my head the levels of players, or underexaggerating them. Maybe I'll try when Brawl comes out.
3. Your learning experience
Like I said, I believe fighting against a human is your best shot. It can be difficult, because depending on who your friends are, you may not have much luck. They may not be the level of gamer you are, and get bored of it, or they may just suck at the game period, which doesn't help. When you have a human, that's equal to your level, it gives you a challenge, and you'll be surprised the things you teach yourself that you find as advanced techs on FAQs later. If you don't, I think your best bet is to watch vids and fight computers. Computers are tediously easy at this point for me, but for a beginner, they'll give you quite a run for a long time, and you'll pick up things.
4. One word of Advice
I've given most the advice I can give, I suppose. One thing that's imperative, in my opinion, is the mastery of dodging, smashing, and items, if you use them. Habitually use the dodge roll, it's easy for people to pick up. From there, you'll slowly master the midair dodge, which always comes in handy. Also, raking up damage can be easy, but surprisingly, it takes a lot of skill to know where and when to perform that perfectly timed smash attack that sends them flying. See what you like. And uh, catching items in midair is huge. It saves you the trouble of having to land on ground to pick it up if you're say, jumping through a platform. If you play item matches like a friend and I, we hardly even ever use the item's functions, they act as effective projectiles.
This was probably way longer than it should've been. Oh well, maybe it'll be more incentive to get in on this great series.