The notes aren't automatically deleted or emailed, there is no time limit.
You can set it up so that notes are assigned/stored with an email account, so they should then be viewable if you logon into the email account on a browser - so you can access them from your computer. On one of my IMAP email accounts (I don't think it's possible with POP accounts), I've set the account so that in Settings > Mail, Contacts, Calendars I have Notes set 'on' - if you tap on the account in Settings to select it you should get a pop-up with 'Notes' as one of the fields, with 'on' displaying the notes associated with the account in the Notes app, and 'off' as a folder under the account in the Mail app.
If you have an email account set 'on' then you should then get an option in Settings > Notes as to what to use as the default account when creating new notes : the iPad or the email account (if you don't have any email accounts set 'on' then you won't get the option, as there is only the iPad to assign new notes to). When you go into the Notes app itself you should also then get an Accounts button at the top of the list of your notes from where you can then select which notes to view - all, iPad, or the email account.
So far I havn't had any issues and havn't lost or accidentally delete any notes, but I'm not creating new notes on a daily basis and what I do have I can easily re-create