According to this neat chart on Wikipedia: http://en.wikipedia.org/wiki/File:PH_Scale.svg
Lemon juice is around 2, and baking soda around 9. Our digestive juices are around a 1, so it's probably safe to go a bit less than 2, but you wouldn't enjoy it. Baking soda is 9, and milk of magnesia is 10...both of which are safe to consume in small quantities, but I wouldn't want to serve a dish in that range.
So...I'd probably stick with 2-9 as an acceptable safe range. As far as taste goes, we're not really wired to enjoy the taste of alkaline. Between 2 and 7 (acid to neutral) is the tasty range.
This would be a bad idea.
Chicken should be cooked 1-2 days after refrigeration according to the USDA and other food safety agencies, and will tend to get noticeably slimy and pungent after 3-4 days in my experience. 5 days is really pushing it.
I understand the rationale for the question - lemon juice can kill the surface bacteria - but that's just the problem, salmonella bacteria don't only exist on the surface of chicken, they're crawling around the interior as well.
In order for this to be considered safe, you would need to be absolutely sure of all the following:
The entire marinade had a pH of significantly less than 4. Lemon juice is considered to have a base pH of 2-3, but that is fresh lemon juice and undiluted. Unless you actually used a pH tester, you cannot be 100% positive of this.
The marinade completely penetrated every part of the chicken that may have been contaminated. This is literally impossible for you to measure.
The chicken itself was relatively fresh before it went into the marinade - unless you got it farm-fresh, you don't know this for a fact, so even if the marinade completely penetrated the chicken, the interior bacteria may have already left behind some nasty protein toxins (which the marinade can't kill because they aren't alive).
If you had said 3 days, maybe 4, I would say, practically, that you'd probably be OK. But any more than that and you're playing with fire.
As the old chef's saying goes: When in doubt, throw it out.
If you are intent on eating it anyway then please, at least be responsible and don't serve it to any guests.
Best Answer
According to (US) Code of Federal Regulations on Citric acid:
Basically, you can eat the stuff by the spoon-full if you can stand the sour. If you're adding a lot compared to how much you think you should have to add, you might consider Phosphoric acid instead. You get a little more bang per mole pH-wise with that, and it too, within limits, is generally regarded as safe (GRAS).