Well, that varies. In Germany it's 8 years before you can apply. I lived in Hong Kong for 4 years, it's extremely difficult to gain citizenship if you aren't ethnically Chinese. Hence the UK policy of 15 years.
As previously stated, I contribute a damn sight more to the UK economy than most, with my frequent trips back!
Switzerland it is 15 years once you have Permit C but it take 5 years to get that at a minimum, so it would take at least 20 years before you could go form being a British citizen to a swiss.
The US is I think 10 years after being a green card holder, but again it is unlikely you will be a permanent resident before 5 years of residency.
So really, the UK's 15 years is probably the minimum that makes sense.
The only grey area I might concede to is dual-citizenship cases, should people be able to vote in multiple countries/ I don't see why they shouldn't because they evidently have ties to both places (normally you can only maintain your original passport if you can prove strong reasons such as family to move back to). I would at least entertain the discussion