How long's a piece of string?
I'm throwing some wildish numbers here, because there are a huge number of variables. But let's say a part 23 normal category single with 4 seats. Two prototypes, one with structural instrumentation, a second with P&HQ instrumentation - 2 TPs, ground staff, 2 FTEs, design support.
Maybe, assuming no fundamental snags, I'd guess 500-1000 hours over 18-24 months. This assumes no significant development flight testing - the design left the drawing board pretty well sorted in other words.
G