"Hi Professor McAwesome,
I just wanted to thank you again for agreeing to write me a letter of recommendation for medical school! It really means a lot to me to have your support as I start this journey, and I am very grateful for it. I just wanted to let you know that I will be submitting my application in the next couple of days, but it won't be marked complete until all of my letters of recommendation are in. I know you're extremely busy, but would you be able to give me any sort of update regarding when you think you might be able to submit it? I only ask because medical school applications are very time sensitive and the earlier my complete application is in, the better! Thank you very much and have a wonderful summer!
Sincerely,
WedgeDawg"
obviously don't use it word for word but you get the idea