Depends.
If it's a feature I tend to only mark it complete when the work has actually been done so that I can add 'Added in 7.2.x' as a response.
If it's a bug I tend to wait until I've received a response to 'Fixed in 7.2.x' or a reasonable amount of time has passed.
If it's an answer to a question I tend to wait until I get a response or until some reasonable amount of time has passed.
Every so often (when I'm bored at work) I will trawl back through the list bringing completion up to date... Using the filter 'Resolution Status = All unresolved' is useful in this process.