Starting June 4th, expiration dates will be assigned to tasks. It will currently be set to one month. This means, if you have a large queue that would take more than one month to process, you will not get credit for all of your candidates.
Basically, you should set your queue to contain no more than 2 or 3 weeks of data. Otherwise there is the possibility you will not get credit for the data processed. Keep in mind that slow machines crunch as few as 200 per month while quick machines crunch a few thousand per month.
To find out how many candidates a node is processing:
Right click on your TSC icon
Click the "statistics" tab
Under candidates, you can see the number processed in the past week (ignore the avg per day). Multiply by 4 to see how many your process in a month. To be safe, I'd take half that number and use that to set your queue size (smaller if you want).
To set the queue size:
Right click on the TSC icon
Set the queue size.