After 1 hr, the number of cells is 1000; after two hours there will be 2000; after 3 hours 4000; and after 4 hours 8000.
The growth formula is f(t)=500*2^t, where t is the time in hours. When t=4, we have 16*500=8000, as we've seen.
The growth to 32,000 cells is 32000/500=64=2^6, so t=6 hours.