I QUIT- Very good question and thread. I will answer based on the age of the Jehovah's Witness.
From reading statistics posted on here last year of how the WT society is losing it's young people ( under 30) and that 2 out of 3 or 66% born in the witnesses leaves eventually I would say of those people under 30 about 33 % probably believe it then. The rest of the young people are staying in to appease older witness relatives and avoid being shunned.
In witness people from age 30 to 50 I would imagine that a higher percentage believe it - my guesstimate would be 50% to 60 % in that age group believe it.
As regards older witnesses aged 60 to over 100 I would estimate about 80 % to 90 % believe the dream still of living in an alleged " paradise". And I only say this from observing older JW members of my family. They want to keep believing it in the worst way- because in their own minds - it's the ONLY hope they've got . Really quite sad actually.
But I'm encouraged by what I see as an enlightening of younger people exiting the cult. As the older members die off - it may be the catalyst for the organization gradually falling to pieces aside from any scandals