What is the meaning of the phrase 'to the extent to which'

Exploring the different senses and phrases of extent may be helpful:

extent OED

c. Phrases: to a certain, great, etc., extent, to the (full) extent of. Hence: the limit to which anything extends; e.g. in to reach the extent.

I have no comment on the grammar in said quote, and am commenting in general on the use of 'to x extent'.

The phrase "to the extent" never means "as long as". "To the extent" is actually a well known expression which is used for adding emphasis on what's being stated. It just means that the speaker reached the point beyond which he/she is unable to tolerate it.

In the following sentence "The Supreme Court on Friday gave the Reserve Bank of India (RBI) “a last oppotortunity” to withdraw a November 2016 Disclosure Policy to the extent to which it stonewalls revelation of every other kind of information under the Right to Information Act, including the list of wilful defaulters and annual inspection reports", the author is saying that the SC gave RBI a last opportunity to withdraw the said policy to the point where it (SC) denies any information under the RTI Act. Here the emphasis is on the level of withdrawal.