I did this but I signed up for their regular checking account I think. Read on what accounts are eligible.
I transfered $50K a while ago(maybe 3-4 yrs ago) and got 25k miles(I think). After I got my miles, I trasnfered the money back to my regular checking account. Now I still have the Fidelity account till now.
So last month, I transferred $50K back to my Fidelity account and guessed what I got another 25K miles. And just last week, I pulled most of that money back to my regular checking account.
I don't know how often they let you do this but I will wait out another yr or 2 and try it again.