Someone has downvoted a lot of my questions or answers.
- What should I do?
- How does the system prevent this?
- How often does the system look for suspicious behavior?
Don't panic. There is a system in place that will prevent abuse of the downvotes.
There are certain patterns that people tend to follow. Stack Exchange has been doing this long enough to have a good feeling for when people are trying to abuse the ability to upvote/downvote, versus when they happen to be doing it by chance.
Usually when people complain about serial downvoting (or even serial upvoting), they intend it to mean that one user (the attacker) is downvoting a lot of questions or answers from another user (the victim).
There are rules in place that detect suspicious voting patterns from the attacker on the victim. If such a situation is detected, the offending votes will be silently removed.
The system looks for suspicious voting patterns as part of a regular, automated process. If you think you have been the target of egregious voting, you can just wait and the system will automatically rectify the situation if it does detect suspicious voting patterns.
Note: A reputation recalc is also performed if the system detects suspicious votes against a user, so your reputation may shift more than expected. More details on reputation recalcs can be found here: How do I audit my reputation?