Hello all! I apologize for the time lag. Am just now recovering from vacation.
First, let me say that the frequency and duration of contact is really going to depend alot on your student's needs.
Here's a suggestion. Set up a regular phone appointment--say 7:00 p.m. every Sunday (Sunday evenings are usually spectacularly dead). Give your student a cell phone or calling card and let them call you. If you don't hear from them, call them and leave a message or voice mail. Just to let them know you're thinking abou them (not that you're checking in on them...).
I don't suggest "surprise" visits (ever). If your student or their campus invites you up for a "Parent's Day," go for it. Otherwise, leave them be. Both you and your student have to adjust to this new situation, and you can't really do that if you're living in each others' pockets. Otherwise, leave visits for special occasions (birthdays, games, holidays).
If you're into self-help, I like "Letting Go: A Parent's Guide to the College Years" by Coburn and Treegar and "I'll Miss You Too" by Margo Woodacre Bane. Parents seem to take to these well.
Good luck!