Thanks Mike. So I did a search on the forums and found the SRS formula listed as - a card is due for review if the score is less than 100 * the number of days since the last review. So although Pleco will mark cards as 'learned' after it reaches a certain score, however these cards will still continue to be due for review - just at longer intervals.
Also I'd just like to clarify something - so on Monday I added a load of new cards, to review with SRS. Later that day I reviewed them, and got most right - marking them with a 6. I noticed this set the score to 600, so I won't see those cards again for another 7 days? Am I using it correctly? that seems like a long gap from first to second review, is that how SRS is supposed to work?