In VOY Homestead, Voyager has detected Talaxian life signs 4.9 lightyears away. In VOY Workforce, Kim finds life signs that are 'less than 3 days away at maximum warp', which for Voyager would probably mean 25 light years (source)
That struck me as being very far away. Their sensors would need to be extremely powerful, even accounting for the incorporated Borg technology.
But have we seen larger distances at which a Starfleet vessel has detected life signs (in canon)?