Well, I know tht most GPSes have something called "cold" and "hot" start-up times. When your GPS has been off for a while (more than a few hours), the GPS has to re-acquire the satellites it was last in contact with.
Since the satellites are constantly circling the Earth, it's very unlikely that any GPS will be able to watch the same ones over any given timespan.
The cold start for my Rino 120 has consistently been around 1 minute, given a clear view of the sky.
If your GPS has only been off for a few minutes, it will be able to pick up the same satellites as before much quicker, usually on the order of 10 seconds or so.
Also, if you turn your GPS off, then travel a long ways (say from the east to west coast), the GPS will take a little longer than usual to start up. The reason being that it will be looking for satellites in a specific place in the sky and big changes in your geographical location throw it off, at least until it can download the almanac and empheresis data from them.
That being said, it's easier to acquire a lock on the satellites if you're outside and have an unobstructed view of the sky and the horizon. Once you get a lock, you can pretty much move around in more confined places, provided you don't like walk into a building or something.
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