I think I have a similar process to the one described by Artem (apart from the popular talk, which I never had the opportunity to do). In any case, I never write a script. Actually, for my first talk in English, during my first year of PhD, I learned the talk by heart, which was terrible, and made me finish the talk in 12 minutes instead of the 20 minutes.
However, I try to carefully prepare my transitions, especially between different parts of the talk. I find it particularly annoying when a speaker finishes a part with a blank, moves to the next slide, and says something like "OK, next section now". Moreover, I usually include in a transition a brief summary of the key points of the previous section, and the motivation to go to the next section, which ensures that I don't forget any important point.
About how early should the preparation be ready, it depends on the kind of talks. For a 20/25 minutes conference talk presenting a paper, I usually already have a pretty clear idea of what I want to present and how to structure it (since I wrote the paper), so I start the slides about a week before the conference, have a decent draft 2 or 3 days before (i.e. the final number of slides, the correct titles), and the final version the day before. As Artem's said, after a while, you can project pretty accurately the duration of a talk from your slides, so there is no need to repeat to make sure you are in time.
For a 45/50 minutes seminar-kind talk, it's a bit trickier, because usually the audience is not the same as the one at a conference, and it's harder not to bore them. So I start preparing the slides about a month before, but only to get the outline, and I start working on the talking part, not so much the slides, to try to find nice ways to present the ideas (such as nice examples, nice analogies).
So, in summary, to answer your questions:
- It is not very common (I actually don't know anyone who does it).
- It can help if the speaker is very nervous, and tends to forget a lot of important things (which often disappears with experience).
- It depends on the experience, and the ability to evaluate your presentation. If you feel very confident, you can start preparing the presentation the night before. But in general, I'd say that something between a week before and the day before is good.
As aeismail mentioned, my answer is quite subjective, and just describe what works for me and for some people I know. However, just one point I would add, concerning the "awkward pauses": I think that the quality of my presentations has tremendously increased when I stopped preparing my presentation around my slides, but the other way around. So I dropped all sentences like "this slide presents ...", "on this slide, we have the definition ...", etc. Actually, I even try not to say the words that are on the slides. I don't know if my presentation are better for the audience, but at least, I feel much more comfortable with them.