If you mean sourdough starter then there is no rule of thumb. Use any amount you wish and things you take into account are taste and timings.
If you mean cake yeast (or fresh yeast) then the conversion is to use 3x as much, in weight, in place of dried with the "usual" amount of dried yeast being anywhere between 0.5 - 1.5% of the flour.
Do you mean sourdough starter or cake yeast?
If you mean sourdough starter then there is no rule of thumb. Use any amount you wish and things you take into account are taste and timings.
If you mean cake yeast (or fresh yeast) then the conversion is to use 3x as much, in weight, in place of dried with the "usual" amount of dried yeast being anywhere between 0.5 - 1.5% of the flour.