Probably never. Especially if you are a thoughtful person.
On the other hand, it does get better. It gets easier, and sometimes you even forget.
Things that help:
- Keep that folder/list (hard copy!) of good things that have happened to you.
- When someone says something good to you, write it down, even by hand, and put it in the folder. Date it, timestamp it, draw a fracking picture in the margin. Put it in your memory.
- When you are giving a good presentation, in the middle, if you can, or afterwards, take a step back and remind yourself of what an awesome job you just did.
- Look at a trainee, the undergrad you are supervising right now, whilst in grad school, and think "this person is doing well because of me"
- There's at least one person who believes in you. Think about that person. Smile at them in your mind.
- Listen to whatever up music you need - the scene from The Two Towers when the elves arrive at Helm's Deep, Eye of the Tiger theme from Rocky, the march from Love for Three Oranges by Prokofiev (this was the music background to a record I heard in Kindergarten for a telling of the Trojan War. I was shocked as adult to learn it was Russian, not Greek, music).
Go ahead and add to the list in comments.