I would schedule ASAP for your internet/cable hooked-up for the day you have the keys in hand. I have friends in the Northern Virginia area that waited 6 weeks for their service to be hooked-up on their new home.
1st,until the actual closing date(or at least up to 3 days prior),anything can happen. I'd be reluctant to request any changes that would incur charges until absolutely sure. 2nd, unless the previous owners lived under a rock,there probably is already a cable line coming from the pole to the house; under those circumstances it will take less than a week to get your provider of choice to come out,give you the boxes necessary and make the hook-up. Remember though...Telephone service is 1st and foremost...without that connection,nothing else will work. Your lawyer is representing you in this transaction...call him/her and request access to the home/property for the purpose of telephone/cable/internet service,prior to closing so that there is no lapse in service. They will contact the sellers and make that a stipulation of closing. As soon as that is done,call and make the appointments. Congratulations and Good luck!