I know that Google measures bounce rates by how fast a user returns to the results page after clicking-through to a website. Roughly what is the maximum duration of the visit for the user to then return for it to be considered a bounce?

i.e. <5 seconds, <30 seconds?

I'm mainly interested as it appears a lot of users clicking through my PPC adverts (Adwords) are bouncing, despite my ads having a high quality score and the page's being entirely related to the adverts copy and at as best tied to what I think user's may be searching for from the key phrases I've selected so the high bounce rate (100% on some keywords) seems a bit strange.

If a bounce isn't determined by time, but simply whether a user returns to the SERP after visiting my site or not after any amount of time that would make more sense but the average duration of visit for my keywords with a 100% bounce rate in GA is 00:00:00, which suggests a user immediately returned to the SERPs, which again, is odd.

Is my GA data being skewed by https or anything like that?

Scratching my head here.