For your reference, this forum post on WinCC flex 2008 http://www.plctalk.net/qanda/showthread.php?t=96524 will answer your question as they both use the same trend view. (TIA Portal is based on WinCC flex 2008, with some nasty exceptions.)
you have up to 999 trend values (properties tab, trend - where you set up the tags) which can be independently set up for each tag. This allows you to trend for up to 999 * sample time.
Anything more than that and you will need to use audit functions. The sample time is variable however so you could lower your sample rate to increase the time you can trend for.