How long does it on average take till one has the NOA, i.e 1 year extension of greencard document, in hand after sending off the I751 application?
Does it usually arrive before the green card expires?

It depends on when you send the I-751 in. If you send the I-751 the day before the greencard expires (which is perfectly legal, but really cutting it close), I'm sure NOA1 won't arrive before expiration.

If you send in the I-751 closer to the beginning of the 90 day window that closes on greencard expiration, then NOA1 will probably arrive well before expiration. Something around two or three weeks or so is fairly typical from the date of mailing the I-751 to the date of receiving NOA1, but of course, like most other immigration processing timeline stuff, this will vary.

Thank you for the fast reply. I really just needed an estimate. Unfortunately I am one of those whose DL will expire on the same date as the greencard will, so I am forced to use the greencard extension document to get me a new DL - which will probably only be valid for one more year then

That`s a good point, I even didn`t think about that. I am in the same situation my Green Card expire the same time as DL, is this based on the green card. So what now , I have to wait first to extand my green card and then I can extend my driving license. IS SIT HOW IT WORKS?
What if I receive the NOA after the GC and DL expire.

I am getting tired to try to put this puzzle together.

Honestly, I don't know how it works. I just know that the last time I renewed my DL (which was one year ago ) , I had to go back to the driving instructor and have him write me a form for the tag office. All he did is look at my resident card. He said without a valid immigration document I wouldn't receive a DL. This is Oklahoma btw, and different states, sometimes even different counties are known to have different requirements, so it would be wise for you to find out how your state goes about DLs.

Anyway, I expect to be able to renew my DL with the NOA1 which is a letter to extent the greencard for one year. If this plan A shouldn't work, plan B consists of calling my state rep up, as he is luckily involved with immigration issues and comes from a small town close to where I live.
If all fails I would be in a very misserable position, as I need a car for everything, since we live in a rural area and the next store is like 15 minues (by car) away....

I am in Nevada
I called DMV, Yes they do need the residency card and our case I have to wait for NOA.