Safe length adjustment on a RRP pole?

Sorry if this has been answered before but I couldn't find it searching.

Is there a generally known safe adjustment length for RRP poles, or minimum required insertion depth? I want to stretch the pole a bit and don't know at which point I should just get new pole inserts. Thanks!
 
Location
Uk
If you look under the pole there is an smal oval cut about 6mm wide and 10mm long on top and bottom part. You can see the pole tube through it.

Don’t extend the tubes past that point.
 
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