There are so many factors it would take a novel size reply for everything involved. Jeanne sums it up nicely as to what it is.
I think you are also looking for how to do it. As Jeanne replied, it's dependent on the weather patterns and time of year for the area. Also the discrete forcasts for the LENGTH of time you are sailing. That's defined by the boat polars...how fast you sail with different wind speeds and directions. Also how fast you sail for your level of comfort and passengers on board.
There are all kinds of weather factors you look for, or look to avoid, and some of these are also area dependent. A low pressure system to the north of Hawai'i is often a good thing, north of New Zealand a bad thing.
Weather patterns can be found in Cornells World Cruising Routes, sailing atlas's of the Pacific and Atlantic and also various web sites. Here's one for the pacific. http://www.pangolin.co.nz/almanac/analysis.php
Actual weather forcasts are available from multiple sources, again dependent on the area involved. Weather gribs are also available (free) from NOAA or sailmail. Weather fax (free) over ssb or internet.
Professional weather routing is also available for a fee. Some of these people are extremely accurate...others not so good.
I had a list of Bob McDavitts weather to look for to make the journey from Tonga to New Zealand. If I find it again I will post it here. It's a comprehensive list of weather patterns which open a "window" to make a safe sail. I think it's available on line but don't have the link.