For example, to configure Piwik to lookup a visitor up to 30 days (30*86400 seconds) in the past, you would set:
[Tracker] window_look_back_for_visitor = 2592000
Note that increasing the value of this setting may have negative performance impacts on your Tracking API requests speed. By default, Piwik looks only in the past 30 minutes. We do this for performance reasons: to keep Piwik tracking fast, even if you track hundreds of thousands or millions of unique visitors per month.