Def ask for it now (or at the end of the week or a few days after the rotation). Have a CV, transcript and step 1 score handy (the latter two if they're good). If you have a personal statement drafted (doubt it and that's fine), give him a copy of that too. Tell the doc to send it to your dean's office once it's finished. If he hasn't finished it by March, then email him w/ a friendly reminder and ask if he'd like to meet again. Then, when he hasn't finished it by the beginning of July, email him once again w/ your ERAS cover sheet and kindly let him know that you're looking to have it submitted by the beginning of August (you really want your application complete at the beginning of September, which means you need everything in by mid-August).