Hi Kirsten, By default it's normally 90 days history for anything that is closed which includes the Tracking tab, the completed tasks, and the forwarded tasks.
You still get all open items of course in the Open tasks tab.
In theory it is controllable through the UWL XML e.g. through the "queryRange" parameter of a view but I haven't across anyone who has tried to do that in practice as yet. I've only played with it myself once and then we used the provided selections for the queryRange.
The no more than 90 days is just to keep the inbox from getting ridiculously large more than anything.
Hope that helps.
Regards,
Jocelyn