I usually don't state when I think protection is gone. I try not to tell people how long a LSP lasts for because it is so subjective. I think it's pretty funny when people say 'This wax lasts 5 weeks and this sealant last 2 months' like it is a fact. As we all know, durability is not set in stone -- every vehicle is going to have different durability. Usually one person will say how long a LSP will last and then 3-4 different people state that same number as a fact becase they read it from that person. Everyone is entitled to their own opinions but taking someone elses word on durability is the worst thing to do because it's unlikely you'll have the same durability.
Anyways, back to the topic. I don't think there is an exact science when it comes to checking durability. I think someone needs to keep a close eye on different things to tell when it would be beneficial to apply another layer of LSP. Beading is one way to tell, but not the only way. If a wax beads nice tight beads for a long time and then they start to get sloppy, it'd be a good idea to apply another coat. But just beacuse the beading let up a little, doesn't mean the LSP isn't protecting anymore.
The same is true with sheeting, if your LSP sheets. The longer the water takes to run off the surface, the more likely the durability is fading.
Another test could be the 'squeaky finger test'. On a freshly polished panel, your finger will squeak when rubbed on the paint. One can do the same to test the LSP, on a clean surface of course. But once again, you'll have to compare your findings to a time earlier with that same LSP and check for variability. I believe that there will always be a 'squeak' no matter what, but if it drags more or is louder after an extended period of time, more LSP is needed. That person has to be the judge though.
If 3 cars were lined up next to each other I'd have a hard time distinguising any visual differences between them if one was polished with no LSP, one with a fresh LSP, and one with a year old LSP. Waxes don't yellow like they used to, so having dead wax left on the paint will be difficult to tell. Then again, I've heard that just because a wax seems to be 'dead', it is still likely protecting the paint ...
Because all of this is complicated, I do a full detail a few times a year. After every other wash or so I'll boost the protection with OCW or a quick layer of wax, depending on the weather and time. I highly doubt anyone has ever accurately measured durability without the use of expensive technology. So when someone offers that a LSP lasts X amount of time, take it with a grain of salt -- it probably isn't accurate. Just my opinions though.
Mak, I'm not sure this answered your questions or not ... if not excuse my ramblings
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But I won't mention any names
