Gmail allows to limit from how many last days it will synchronize the messages. E.g. from one day, two, etc.
What is the default setting? I think this is phone independent, but I ask about Samsung Galaxy S, and SII.
4 days I believe, up till Gingerbread. I have Ice Cream Sandwich (ICS) and it's currently set at 30 days and I haven't changed it on my device since buying it so I suspect ICS has is set to 30 days as default.
If you look at: http://support.google.com/mobile/bin/answer.py?hl=en&answer=168499 (along with the other links I post below) you can see it mentions a 4 day sync, these posts seem to be referring to Gingerbread and below.
EDIT: After Googling around for a bit I think that 30 days is definitely just in ICS. A few people on various sites mention a 4 day sync which should have been Gingerbread and below. (Note that most of these sources are relatively poor but they mention the 4 day limit, hence why I've posted them)
This default depends on your Android version. The default setting used to 4 days but with ICS (might be honeycomb, I can't comfirm) it was changed to 30 days.