If it's side-shooting then as said above it depends on the type - bush are left with all the side shoots on, but large fruited indeterminate varieties are usually side shooted and the top stopped at some point - here in about August, but when you judge that any new fruits to form will not have time to ripen before the end of the season. I grow a variety called Sakura which is a cherry type which will grow to about 8' tall and has lots and lots of trusses. I take out the side shoots low down then allow about half a dozen to grow on. I stake all the side shoots and get a much larger crop than I would if I just let the main stem grow and this variety is still producing edible tomatoes into December with protection (polytunnel plus lots of horticultural fleece at night). So it depends an your variety and local conditions.