A Ti frame that is made and welded of a proper alloy using proper welding techniques(emphasis on proper in both cases) can easily be an heirloom. I have a '96 vintage Litespeed Catalyst with about 75,000 miles on it and it is still in perfect shape and i know of other litespeeds with well over 100,000 miles so mine isn't a rarity. So a good frame by a good maker will last nearly indefinitely. Litespeed, Titus, Moots, Seven and others make very durable Ti frames.

That said, a poor choice of alloy (CP grade instead of 3Al/2.5V or 6Al/4V) and/or welding without adequate attention to inert gas shielding can and has produced brittle, crack-prone frames. One of the first commercial Ti frame, the Teledyne Titan from the early 70's was made of "standard diameter" CP grade titanium and developed a well-deserved reputation for early failure. Also, some '90's Russian frames made of CP Ti and without adequate welding care were also very failure prone.