I believe the length of time spent out of the UK could affect future visa applications.
Yes, it can, but as it says, it depends on the nature of the absence from the UK, whether or not your spouse was outside the UK with you, and how long you were away for.
For example, if you spent several months a year in the US, while your spouse stayed in the UK, that would imply that you were not in a genuine relationship and were not genuinely living in the UK (you were just using your spouse to get the visa).
However, if you and your spouse split your time between the UK and the US, and you always lived together in the same country during this time, this may not count against you because the relationship has obviously been subsisting for the entire time. I remember someone here on UK-Y was granted ILR with no issues (under the old rules), despite spending 6 months of the year in the UK and 6 months in France... because she and her husband moved between the two countries together and were in a subsisting relationship for the whole period.
I know for citizenship, you must have spent no more than 450 days outside the UK during 5 years, and no more than 90 days outside the UK in the last 12 months when you apply.
Just to clarify, the 5 years is only if you're NOT married to a UK citizen.
If you are married to a UK citizen, then the qualifying period for citizenship is 3 years and you cannot have been out of the UK for more than 270 days in the 3 years, with no more than 90 days in the final 12 months.
So, while it now takes 5 years to get ILR under the new rules (and 2 years if you're on the old rules), it's still only 3 years residency to qualify for citizenship, even though you have to wait 5 years before you can apply... therefore the first 2 years on the spousal visa don't count towards the residency calculation for citizenship.