I have 120psi of compression how low can It go before a rebuild. Thank you

Compression is not an accurate method of determining the condition of a TWO STROKE ENGINE.

Measuring compression only gives an indication of how well the rings are sealing and does not tell you how loose the piston is..

A loose piston will break and turn your engine into junk where worn rings will not usually damage the engine but just cause the power to be a little weaker.