The ratio I had to meet in private industry was a two year payback.
In advising various public agencies, I regularly advise some of them on the theoretical benefits and problems with borrowing money for some capital projects. If one is a manager of a City owned electric utility and one is building a new substation that has a 20 to 50 year life, there are some good reasons to issue 20 year bonds to pay for the project.
One reason is that if the project were paid from current revenues by current rate payers their rates would be higher now than they would need to be. Some of them might die and no live to see the full benefits of the project over the next 20 to 30 years. For most public agencies borrowing costs are pretty reasonable (unlike credit cards). Funding public capital projects is in some respects a "cross-generational" subsidy. If folks want to do this kind of thing it is fine and actually results in lower societal costs in the long run, but it is a subsidy to the future.
At least it is not a reverse generational subsidy like the way Social Security is lining up to be.
I ususally advise my clients on the downside of borrowing too much for capital projects. One of the biggest, is that you never know about the future of the local economy and what you can afford in the future. Another has to do with what other kinds of infrastructure will be needed that you may not be able to affort because you are maxed out on bonds.