nah, time handicaps don't work well. if the nm is decent at rapid he will probably still score very high, maybe 2000 leveled.
at nationals i've spent around 15 minutes per game in a 90 minute time control game and had no problems vs 2100s, still won nearly every game.
Put simply, If we take a baseline master, at what point could we say that if the High-C to Low-B player had approximately 90 minutes what amount of time could we give the master to ensure that they were playing at approximately that level as well?
My guess would be around 10 minutes.
I played a Master once in a bishop, and pawn ending. I had about 10 minutes left, and he had the 5 second delay. Thats all it took for him to beat me. A 10 minute handicap would be nothing for a Master.
At our club we will occasionally have handicapped 20 minute tournament games. If i remember correctly, the time difference with a 600 point ratying difference would be either 18-2, or 16-4
When I was 1557 I drew two NMs in a tourney :) Two things contributed: (1) My rating had not caught up to my playing strength since I'd mostly played in scholastics, and (2) the NMs underestimated me and so played very quickly in the opening. Because they played quickly, I got better middle game positions. Then they slowed down, but I still managed to draw.
An overconfident NM could have bad results against a 1600 player. http://www.chess.com/blog/NMKane/take-all-games-seriously---how-even-gms-throw-games-away
But if you had a 1600 who wasn't underrated vs. an NM who is not overconfident, the NM could win a lot of games with a lot less time because of pattern recognition. Occasionally the 1600 would win because of something the NM overlooked. Mostly it depends on the position.
Put simply, If we take a baseline master, at what point could we say that if the High-C to Low-B player had approximately 90 minutes what amount of time could we give the master to ensure that they were playing at approximately that level as well?
My guess would be around 10 minutes.