No, there is no "conversion rate" because in baking there is no fixed ratio of yeast to flour or other carbohydrates from the start.
Simply put, the time your dough needs to rise is a function of yeast's growth rate over time - if you start with little yeast, you need more time until you reach the same result.
The growth rate can be influenced by ambient temperature, available food (sugar or flour) and inhibiting factors like salt.
You might be tempted to add sugar or more yeast to "speed things up", but frankly, you probably don't want to. Too much yeast will result in a distinct "yeasty" taste in your finished bread, too fast a rise results in uneven bread structure and "flat" taste. With yeast, slow and steady is usually the better choice.
So yes, you may absolutely decrease the amount of yeast (at least if you start with a typical recipe from the Internet or similar sources), especially if you add sugar. I usually at least halve the amount without batting an eye, but whatever you do:
Don't judge rising times by a looking at your watch, but by looking at the volume of your dough.
A good recipe should always define rising by volume, e.g. "until doubled in volume" or similar, not just give a fixed time.