The current schools of thought on being able to remember passwords are

A) use a passphrase I.e. I go shopping twice a week every week. You'd then take the first letter from each word for your password so it'd become "igstawew" which is really random but memorable.

B) write them down in rl.

Have different passwords for everywhere. Use symbols and numbers.

How frequently should you change them? How critical is it if your account gets breached? How likely is that to happen?

You don't want to be routinely changing too many passwords, you're more likely to stop bothering.