On average an RFE is issued for about 50% of the cases. This not very scientific, just an educated guess from looking at the info in Rupnet.
As for time from receipt of evidence to adjudication, anywhere from 2 weeks to 6 months or longer. Again check Rupnet by sorting the RFE column and AD column.
For me personally, it took 5 weeks from receipt (Dec. 26th) to adjudication (Jan. 30th).
Hope this helps.