Do I write a letter now or later?

I asked a professor to write me a letter of recommendation for an application. She did it, and turned it in in a timely fashion. It was submitted last Friday, September 05th. I was planning on writing her a letter to thank her, but I’m not exactly sure when. I talk to her a lot and see her in the halls, so I thought it’d be weird to write her a letter if I see her so much anyway. I feel like writing a letter of appreciation is still necessary in social context, especially after she wrote one for me. I don’t want to buy a card or anything, I just prefer a handwritten letter. Is that socially okay? Should I wait until the end of the semester when I’m leaving or write it ASAP and leave it on her door?
Please feel free to share any thoughts you have on any of this.