Willow and Xander say they didn't see any vamps in S1-2. During the S2-3 summer, they say that they 10 vampires over about three months, which is maybe one every nine or ten days - so while they were out there, it was still a pretty light time.

Otherwise, there's no real talk of what the level of activity was in the S3-4, S4-5 and S6-7 summers. Whether it was pretty much no activity, or if it was just some low-level activity.

The S5-6 Summer is really the only one where stuff goes down - because it's really the only one where it's a plot point to have something happen.