There is a 10 minutes time limit for every run. This normally is enough to process thousands of records (depending on which extension). This limit does not necessarily mean that you cannot run the extension again. You can run the extension again if there are more records to be processed. We also have a daily limit of 5 hours of total run-time. This limit will reset every 12AM UTC.
The extension doesn't remember the records it processed, but it will process any records where the target field is empty.