When it get too stringy etc. I have used it right up to the frosts.

I'd suggest you stop using now... the remaining leaves need to feed the crown for next year. Get greedy this year and you'll lose out in subsequent years.

I find the stringy aspect is the main reason for stopping harvesting. though having said that I have had some small red stems this last week. yum :o) there are 7/8 large stems on each crown so I reckon that will do them well for next year.

To clarify - I try to never take more than one stalk at a time from each crown so weakening isn't a problem. If you only have a small clump best take Moongrower's advice.
And just before the frost I've sometimes taken all that was reasonable to make jam - sadly nobody to eat it nowadays...