As for question two, a simple thought experiment helps shed some initial light. Imagine you are asked to learn a set of stimuli that are presented on a screen for 1 s each. Surely you'll get a big mnemonic boost for those items if you instead study them for 2 s each.
But then imagine you have studied an item for 60 seconds -- can you imagine one extra second will make a big difference? The relationship between study time and eventual performance surely begins to plateau.
In cognitive psychology is an interesting finding called the labor in vain effect, which suggests that when people are given an unlimited time to study, occasionally they will expend a great deal of effort on items which they'll never learn.
Nelson, T. O., & Leonesio, R. J. (1988). Allocation of self-paced study time and the "labor-in-vain-effect." Journal of Experimental Psychology: Learning, Memory, and Cognition, 14, 676-686. [PDF]