When and if you set up the voicemail box the greeting will be as long as you make it.
All sound files are below /var/lib/asterisk/sounds , the text of each of them is listed at http://www.voip-info.org/wiki/view/Asterisk+sound+files, feel free to rerecord them to suit.
Read the Wiki and search the fora for answers already given or start new threads for off topic subjects but for most small users I would suggest you use your gmail account as a relay, there are many recipes to do that in postfix out there, some even here in these fora.