How much regular yeast replaces natural leaven?

Not sure what you are after but you can't just replace one for the other.   Is it that you do not have a starter going from which to construct a levain? Or, is it the "sour" in sourdough that you want to avoid?  But even before answering those presumed questions, my best advice is to use another recipe that calls for yeast. If you have no starter, then look for a recipe that calls for a yeast-based pre-ferment like a biga or a poolish. If it is the "sour" part of sourdough then this is a long complex question that is best answered by suggesting that you read a book like Tartine Breads. Sourdough is a terrible common name for natural yeast that does not have to produce sour bread if you manage the levain properly. So, why do you want to do this?

If you forgot to elaborate your starter or for some other reason had to get started on a dough right away with quick rises you could go with 7g instant yeast to 500g flour, reduce gluten -- sustitute some of the bread flour for AP flour unless you like a chewy crumb.  Be prepared for a carb rush when eating and do not call it bread from a sourdough recipe.  

If you wanted the long rise, reduce to just 1g to 2g yeast for 1000g flour.  You may have to add the flour and water used in the starter into the dough recipe to get the finished volume you desire.  

The flavour will not be as complex as when taking the starter ingredients and add just a pinch of yeast for an overnight poolish (fermenting 8 to 16hrs) before making the dough.  

that before 1875 or so, all breads were made with some kind of natural yeast; salt rising, all kinds of YW's, beer barms, wine foams. Witches yeast, and all the various kinds of SD starters made from all kinds of liquids, flours and spices.  All of these breads were converted to commercial yeast very quickly and easily. 

There really hasn't been any 'new' breads for just about forever.  There have been so many breads formulated over the last 6,000 years minimum by millions and millions of bakers over the years there isn't really anything new to discover that some baker didn't do a thousand years ago.

So you can convert any commercial yeast bread back to the SD it came from or convert any SD bread back to commercial yeast.  All you have to do is follow Mini Oven's lead and instructions to make a fast bread with less flavor and lots of yeast, or slow the process down to get better flavor using a biga using a small amount of yeast or make a poolish using a tiny amount of yeast which takes the longest.  Bread bakers have been doing this routinely since 1875 - I know I have!