At my current company call is about every 10 weeks. The only calls I've gotten are for batch processes that aren't automated and have regular problems for which there are standard fix scripts. Don't ask why the regular problems haven't been fixed :-D. That's only 5 times a week and at hours you are normally awake anyway.

At the last company I had call duties for we started with 9 in the IT department and ended up with 5 before my position was eliminated. We were on call for everything IT, from PC to network to Exchange to SQL Server. 99% of the calls were non-SQL related and had documented solutions. It still sucked at 3am. Averaged maybe 4-5 calls a week.