accordlegend,
I'm pretty sure the Z1, Z2, Z5, Z6, and Z7 do not come with any "seal" on the top of the bottle. Therefore, IMHO, they really are "opened" as soon as they are manufactured, and the shelf life starts at that point! The Z16 tire gloss does have a little red cap under the lid (probably to prevent spillage more than seal).
To be on the safe side, you probably would want to check with Sal. I'm sure he would be more than happy to answer your questions. Personally, to be on the safe side, I only buy enough for one years worth of detailing!
Also, I don't know how long you've been using "Z", but here is a "rough" guide to what I use a year. These numbers are based upon about 8 months of detailing during the year, with a wash and QD about once a week with fairly frequent coats of Z. I have an F-150 SuperCrew which is a big vehicle. You will need to judge in proportion to the size of you vehicle.
1 of the "tiny" bottles of ZFX.
About 1/4 bottle of Z1 (still use, but only occasionally)
1/2 bottle of Z2 (use more Z5 now)
1 bottle of Z5
3 bottles of Z7
4 bottles of Z6
1 bottle of Z16