A too long salt will not reduce security. A too short salt will reduce security.
As the salt gets longer security will improve. At some point you will cross a boundary, where you start getting diminishing returns on increasing salt length. And eventually you will cross another boundary, where a longer salt does not add any security whatsoever.
However since a longer salt doesn't have any significant cost, there is little reason to stop increasing the length until you reach the second boundary. Even if you were to increase the salt length beyond that, the only drawback is minor extra cost in terms of storage and processing time.
Sadly salts are more often generated with a length below the lower of the two thresholds.
So what are the two thresholds?
The lower threshold is determined by the number of users and the purpose of the salt, which is to be different for every password ever chosen by any user. If we take some conservative estimates, the number of users worldwide is less than 10^10, and each user has passwords for less than 10^2 systems and over their life time changes this password less than 10^3 times. That means in total there will be less than 10^15 passwords. Given this, one might erroneously conclude that 50 bits of entropy in the salt is enough to guarantee that there will never be two salts repeating, but due to the birthday paradox we have to double that number.
So once the salt grows longer than 100 bits, we start seeing diminishing returns in terms of added security. The 100 bits threshold involved a lot of guessing. The next threshold involves much less guessing. More than 100 bits of entropy in the salt still improves the security, just not by much.
Once the salt grows to be longer than the output of the underlying hash function there is however no added security from making it any longer. What the threshold is depends on the hash function, these range from 128 to 512 bits for typical hashes.
Remember that salts are usually generated with 6 bits of entropy per character, so you would need 86 characters of salt to reach the 512 bits of entropy.