Good question... And here's the answer...
First, there is always an end. You even said so yourself: 10-15-20 years.
But even without an end, there's a metric called Internal Rate of Return (IRR) that can tell you the estimated compounded rate of return over the life of the deal, even if the deal goes on forever.
This is because there's something called Time Value of Money, where money earned later is worth less. Money earned 15, 20 or 100 years in the future is great, but factors much less (and eventually not at all) on the returns than money earned in the short term.
And here's a cool little secret for you... You talk about getting your principal back sooner and future money being "infinite profits," but from a math standpoint, that's irrelevant.
Here's an example:
Let's say you put $100,000 into a deal, and after 2 years you get $100,000 back. Then you get $10,000 a year for the next 10 years.
Overall, you invested $100,000 and you get $200,000 back. Does it matter if the first $100,000 that you got back after 2 years was the return your original investment or whether the $100,000 you got back over the next 10 years was the return of your original investment?
It doesn't matter. Saying you're getting your original investment back first results in the exact same mathematical returns as saying you're getting your profits first and your investment back later.
In the example above, all that matters is that you received $100,000 after 2 years and $10,000 a year after that. Doesn't matter which was the profit and which was the return on capital. The return metrics are all the same.
If you want to better understanding of any of that, check out the latest book I published with BiggerPockets. It digs into all of this and a whole lot more...
https://store.biggerpockets.co...