I don't know if there is a max length.. but I think around 50,000 frames.
The longest timelines I've seen are when people embed video and then the timeline has content out to 5000+ frames. In general, use of the timeline should be kept to a minimum, at least I've heard. Most people recommend actionscript to due the bulk of the work with the compliment of the timeline.
When the timeline is really long you can type the frame you want to goto and that might get you there more quickly. Depending on the system very long timelines can really bog things down.