How long do you reckon it takes (like in years) before one who takes aikido is an effective combatent in the art?

453 days

Short answer: As long as it takes.

Longer answer: It depends.

Much longer answer: It depends on the individual, their learning capacity, number of times a week they practice, their attitude, etc. etc. etc.

Some take anywhere between 8-10 years, others take 20-30, some take a whole lifetime.

Not just in aikido, any martial art.

BTW, don't you mean "exponent", not "combatent" [sic]?

hmmm...
I guess the answer is rather arbutrary,
I'm curious becuase I eventually want to take a martial art that is very long term, like Tai Chi.

But I sort of want to get this macho "need to be able to kick a**" bug out of my system, so I after I get some training
(5-10 years[?]) in aikido I can really concentrate on an art that has a extremely philosophical focus.

Of course many are probably wondering: "if you just want to fight well, why not take somthing like boxing?"

Many reasons. First of all being I'm already hooked on Aikido itself; I love the flowing movment, and its subtlty. And partly because your average lay-street fighter (speaking generically there) probably going to use his only tool: conventional strength. Which means attacks are going to be along the line of Tackles, Hooks and Jabs. All of these feel like effective menuveurs to the untrained (been there) but unless you know what your doing, they're are rather useless (as with many "styles" of combat).

Long and the short of it is that Aikido is perfect against this kind of fighting. Oh, and also because everyone punches, lets break the trend...

How many times a week do you recomend for classes?

Yes, the answer is absolutely arbitrary and even then is meaningless. You said you were hook on Aikido, so just do it, enjoy the hell out of it and live a great life.

As for how often, how often can you go? Are you married? Do you have kids? What about a job?

For years I trained 5 or 6 days a week. Them days are over for me. I have a wife and a son and they are far more important to me than any martial art will ever be. I'm fortunate that three days a week fits very well into my lifestyle and four days a week is pretty easy as well. No way, no how could I fit in 5 except on very special occasions.

Just figure out what's important in your life (hopefully things you enjoy) and then go do it!

Chris