I've kept relations with them outside of the classroom and believe I left a good impression on them in the classroom as well. One of them is a M.D. and is a member of NIH. He invited me to his research lab, and I helped him set up a system where he could upload his lectures online for his students. The other is a science professor who did research at a med school. I've made biochemistry notes for him, which he uploaded online for all of his students to use and gave me credit.
I wanted to ask them for a letter now, get them to upload it on interfolio, giving them more than enough time to write the letter. I was just wondering if this is too soon, will it be ok to use the letters for the next app cycle?