When (theoretically only, of course) being away too long from SO, will my reputation score decrease some day due to the fact of being idle/away for too long?
No...I presume at some point in the far future your account might be deactivated perhaps...but you do not lose reputation over time through in-activity
No. However, because all active members continue to gain reputation, your score's relative value will decrease over time.
To complement David's answer, there might be some decrease in rep due to deleted accounts or deleted/moved questions in which you have answers, but there is no time-dependent reduction. All rep count is based on your total activity, not the time you've been active. Note that this rep reduction could very well be offset by upvotes in your standing questions and answers.
As someone who did what you describe, for unavoidable health reasons, my experience is that reputation doesn't noticeably decline.
In fact, if you've answered enough questions, it might slowly creep up as people discover your good answers and vote them up.
(The other side of the coin is that you might be voted down for hastily composed answers)
Ideally, it should be possible to reach a critical mass of answers which can sustain your reputation score without further effort.
Your reputation doesn't go down on it's own, except for some odd circumstances - like if you've somehow gained a lot of rep through duplicate questions that eventually get deleted.
On the contrary if you have a good base (maybe the low thousands) you'll probably see your rep go up. I think I average about 5 rep a day worth of "interest" on the answers I've already "deposited". And a lot of it is from the same questions - answer a question that is asked a lot and you'll get repeated hits from other SO users coming to your solution and voting it up.
Given that I (~5000 rep) already earn ~5 rep/day in interest (0.1%), I have to wonder what some of the top users get. Those in the 200K reputation range could well be getting almost all of their 200 rep a day just from old answers.