@Mr:
There's been no confirmation, we can calculate the time they take on the islands, on average, but we have no idea how much they take to go from island to island.
I'd say 1 year is about right.
because of what? because you said so?
Also, do you really think pirates to go from island to island smoothly without any confrontation, or staying at some places for extended periods of time?
The time required could very easily be much shorter then one year and vice versa, don't just pick one year because "it is about right", there has been nothing to suggest that. Little garden is not the only island on the Grand line that requires an extended period of stay, and there has been nothing that supports that it is the only island.
If we are to assume all prehistoric islands require an extended period of stay, this would actually go against your one year theory. Since Nami is asked "were you on a prehistoric island?" or something like that, which suggests there are multiple prehistoric islands, instead of "were you on little garden?" as far as I remember.