If you're interested in the point at which a time signature no longer makes sense, or at least where the composer decided not to use one in an otherwise (more or less) conventional score, consider this excerpt from the first page of Tristan Murail's "Les Travaux et les Jours" for piano. (Information and score preview here; listen to part I here.)
The sense of pulse and meter in this composition is so elastic that the composer has resorted to non-standard methods to notate the rhythm: there is no time signature, there are no bars (though there is an indication of equal time segments at the top), the note beams indicate a gradual shift from 1/8 to 1/16 to 1/32 note lengths and back, and the horizontal distance between the notes is used as an indication of the pulse.
Also, at some points in the score, the length of pauses is indicated in seconds, and there is a pedal point indicated as "very long". This is mixed with more standard indications of tempo, such as "meno presto", "accel. poco a poco", "rall." and even "♪=66-70".