hi-- these may be of use

Intertemporal Decision Making,  Laibson

Hyperbolic Discounting and Consumption. Harris & Laibson

I'm not sure if it's exactly what you're looking for, but it gives you some basic analytical structures.

Dasgupta P and Maskin E. 2005. Uncertainty and Hyperbolic Discounting. The American Economic Review. 95:4:1290-1299.

And good context for discounting can be found in Heal G. 2007. Discounting: a review of the basic economics. The University of Chicago Law Review. 74:1:59-77.

Also check Laibson D. 1997. Golden eggs and hyperbolic discounting. The Quarterly Journal of Economics. 112:2:443-477.

I wouldn't get so stuck on precise forms unless you have data to work with. The article does a good job of motivating the idea that real people calculated discount rates in a non-linear manner when going from 1 year to 5 years, to 50 years, etc. You can use an actuarial (non-parametric) approach if it is difficult to defend a specific functional form in the model. This would allow you to taper from, say, 10% in year 1 to 2-3% in year 50 without having to defend a simplistic functional form.

Also refer to Weitzman, Weizman ML. Just keep discounting, but … . pp. 23-31 in Discounting equity ed. PR Portney and JP Weyant. Resources for the Future. Wahsington. 1999. - he polls economists and finds that many agree on a long term discount rate of just a couple percent, even though we routinely make short term calculations with the assumption of 6-8%. As an alternative to hyperbolic discounting, he also wrote Weitzman ML. 2001. Gamma Discounting. American Economic Review. 91:1:260-271.

So-called "cultural discount rates" are also a big deal for many considerations (why don't they want to cut down the forest when we "know" it's profitable? - Faustmann formulas are a good basis to make comparisons on this kind of problem, and that goes back to the 1850s (?) for forestry analysis.) Oberhoffer, Tom. 1989. The Cultural Discount Rate, Social Contracts, and Intergenerational Tension. Social Science Quarterly. 70:4, 858.

Here's another one.

Rubinstein A. Nov 2003. “Economics and psychology”? The case of hyperbolic discounting. International Economic Review. 44:4:1207-1216.

A quote from Rubenstein gives an idea of common experimental methods: " Ainslie & Haslam (1992) – “a majority of subjects say they would prefer to have a prize of a $100 certified check immediately over a $200 certified check that could not be cashed before 2 years: the same people do not prefer a $100 certified check that could be cashed in 6 years to a $200 certified check that could be cashed in 8 years.” "

I was trying to motivate the form of the discount rate for a soil conservation model, but got wrapped up in other details and decided to keep the math simple by using the classical approach, in order to more effectively highlight other issues.

I do believe that this is significantly understudied and this implies great faults in most analyses which extend beyond the 5-10 year time frame. It's been a few years, so take the above comments with a grain of salt, but from the articles suggested above, I think you will find that there is strong empirical and theoretical support for a need to develop better ways to model the ways in which economists discount future flow of profits/gains/wellbeing.

I think this falls under behavioural economics. But grad students in research labs don't always make the same decisions as real people. Consider the case of energy efficient appliances purchased by people who are not expected to face credit constraints. This will spin your mind backwards and forwards - sometimes people exhibit discount rates in excess of hundreds of percent while the lock GICs in at 3%!

If you cannot access these articles, please message me.

If a person does not face the same real interest rate for different periods, then one would expect the discount rate to vary too. Think about this in the context of a Hirshleifer 2 period model. Note how the discount rate emerges as a reflection of the tangency between an indifference curve and a credit market opportunity line. Of course, limitations on borrowing, i.e., credit constraints, essentially chop off the credit market opportunity line and potentially destroy the linkage between the discount rate and the interest rate. More realistically, the credit market opportunity line is probably a concave function instead of being linear and then ending abruptly. If this is the case, the discount rate is not equal to the interest rate, but it is related to it. 

Alternatively, hold the discount rate constant, but allow the returns in various periods to have different distributions, i.e., use monte carlo to discount future returns. I use triangular distributions, because the three parameters of the distribution are easy to explain to people: the best case, the worst case and the most likely case. More precisely, I make the changes in returns from period to period have distributions so that returns evolve reasonably through time.