Is it possible to reduce the amount of yeast and increase the rise times for leavened doughs? If so, is there a simple method to calculate the changes? Does 1/2 the yeast mean twice the rise-time?
It is definitely possible. However, there are many, many factors that are going to guide yeast activity: sugar levels, water temperature, gluten development, etc. If I can, I normally cut back the yeast and just keep an eye on my dough, the less yeast, the more the flavor can develop.
In optimal conditions, the yeast cells in bread dough can double their number in about 100 minutes to 2 hours. And by optimal, I mean like in a lab with perfect ventilation and temperature control @ 86°F/30°C . In a kitchen there are many other factors that can influence this (e.g. temperature of room, salt and sugar levels of the dough, presence of other yeasts in the air, etc...) so that 2 hour number will only go higher depending on your setup.
But if you use that 2 hour number as a guide, each halving of the yeast in the recipe will add 2 hours or more to your rise time. Note that this rule of thumb won't work if you are looking to do a refrigerated or lower temperature rise.