First, your password manager is a (soft) 2-factor authentication. You need a file you have and some password you know. This makes it already much more secure than a plain password.
When you now assume somebody got you password file, there is the (weaker) password left. And now you get a paradox: Assuming somebody can copy your password file without you noticing but does not get your password, changing your password actually weakens the security.
The reason is, that the attacker has a collection of your password files (e.g. 10 versions with different master passwords). When he's now brute-forcing your password, he just tests each password on all files and the chance to get it right is 10 times bigger than brute-forcing a single file.
Note that the same is true for most disk encryptions, which do not re-encrypt everything when you change your password. An attacker who is able to copy the header (i.e. LUKS) can brute-force the master key in the header even when you changed the passphrase.
The result is, that changing a password when you suspect somebody may get an encrypted file does not help, if there is any chance that he already got the file. If you suspect he got the password but not the file, change it. As regular changes are making things worse, choose a good password and keep it. You may also consider physical security for your password database like putting it on a usb stick, if you can make sure you will not lose it.
Regarding the question, if a good salt and hash stretching would not prevent the problem:
When you change your password and the attacker starts brute-forcing both, you have two options:
- The new password is weaker: The attacker will find the new password faster than the old one and the overall security is weakened.
- The new password is stronger: The security does not change, as the attacker will find the password of the first file just as if you never changed your password.
This means, you cannot make the security stronger. Either it keeps the same or it will even become weaker.
Hashing the password more often makes the brute-forcing slower, but only by a constant factor (number of files available). Compared to an exponential factor when adding more characters when choosing one good passphrase, this is not the bottleneck when cracking a passphrase.
I assumed here, that you only change the master password. If you change all stored passwords when you changed your master password, this does not apply.
Conclusion: Choose one good long password at the beginning and keep it. And make sure nobody gets access to your password database.