Simply find out how long they have been instructing.
When I looked back on my first 6 months I shuddered at how cack handed I was at the beginning, after the first year I looked back at me with 6 months and marvelled at how little I knew, after 2 years I realised there was a lot more to instructing than meets the eye, when I finally left instructing this year after 5 years and being an Approved IRI I *knew* how much I still had to learn...
So if they say 6 weeks perhaps its time to gently make your excuses.
However. To blow all that out of the water I would say that boredom and contempt for the job are the root cause why so many instructors give very poor tuition and at least your green but keen new starter avoids this.
Good luck,
WWW