To my knowledge, S100 has not/is not going to be discontinued. As mentioned in that Autopia thread, S100 is also known as P21S. It's the same product but the P21S is about $10 more per jar than S100. Same product, different packaging, different price.
A jar will last you quite a while. Probably 20-25 waxes. You really don't use a whole lot when you apply. Less is really more. Since you wipe it off right after you apply it, you want to put it on in a thin layer. All of the excess gets wiped off so using more than a thin layer would just be wasteful. Because S100 is a true carnuba wax, you only apply it every 2 or 3 weeks. Because I don't drive my car very much, I usually apply a coat of S100 every 3 weeks in the summer. In between coats, I use a quick detailer called Pinnacle Crystal Mist. It is a carnuba quick detailer that will maintain that just waxed look and wil allow each coat of S100 to last longet before it needs re-applied. You can find any of these products (and the ones they talk about on Autopia) at
www.properautocare.com
I have had my S100 for about a year now. Same jar and it's only half gone (~15 waxes). It has been in temperatures ranging from room temperature all the way down to about 35* and it still performs as good as it did the first time I applied it. If you are worried about it 'expiring' or getting old, you could always throw it in the fridge in-between waxes which might make it last longer but if it just sits out, you shouldn't have to worry about it going bad or expiring.
One more thing, I would also recommend a microfiber towel to remove the S100 with (and do the rest of your detailing with). Microfiber is softer than cotton and WILL NOT SCRATCH the clearcoat or leave little swirl marks. For towels, I recommend
www.pakshak.com Ranney has some great prices and his customer service is outstanding!
After spending some time on Autopia, you will learn alot of new information. Washing, drying, polishing, waxing, how to wash your towels, products to use, etc. It's the best site for detailing that I have found and the information isn't biased toward one company or another. Everyone can give their opinion on what works and what doesn't.