Hi there! It all depends on how meaningful your interaction with your PI has been. Remember that, just like a statement of purpose or personal essay, letters of recommendation should show rather than tell. In other words, your referees need to mention specific attributes that they have observed in you and provide context or specific examples of those attributes. They should also mention the unique contributions that you would bring the program, and they do this by describing experiences you have had and the learning that resulted from them. The skills that you have developed will hopefully align with those that the program values, and in this way, the admissions committee knows if you will be able to contribute to the program and be successful once admitted. So, if you think you have already had a chance to show your PI those essential qualities that make you unique, you can ask for the letter. Otherwise, you may want to wait a while and, in the meantime, devote your time to building a meaningful relationship with your PI and team members so, when the time comes, your PI will have plenty of things to say about you. Remember that you want a strong letter from them, so you probably need to give them time for that to happen. Hope this helps!
