Probably around 500 hours if there is even one established! My 1943 Lycoming has a TBO of about 700 hours. I had a bit over 2500 hours on the last major when I pulled the cylinders off it a couple of years ago. All nine cylinders checked out within new cylinder tolerance. I replaced one exhaust valve and one valve guide and reused all nine cylinders.
TBO have virtually NO meaning unless an airplane is used under Part 135 or Part 121. The hours portion of the TBO is particularly meaningless for private use for most people. A TBO of 2000 hours or 12 years, whichever comes first, will mean that most new Lycoming engines will reach TBO with far less than 2000 hours on them.
The accepted practice for private aircraft is to run the engine until it shows need for service or overhaul. The compression will be shot, the valves will leak, or it will not develop anywhere near the rated power, then overhaul it.
I own a Piper Apache that has 600 hours on the engines since new. They are the original engines. These low time engines were pickled by Piper for several years, but they probably reached their "TBO" forty years ago. They are still "low time engines". :-)
Do not be misled by claims of low time on older engines. If it checks out as a good engine, it is probably a good engine. Do not expect the impossible. Remember, new cars of that vintage were expected to be ready for at least a valve job after about 40,000 miles.