Sure... acceleration due to gravity is 10 m/s/s, meaning that your
distance travelled is (1/2)*(10)*(t)*(t), where t is time in seconds.
Solving 100 = 5*t*t, I get t = 4.5 seconds, approximately.
Since you will need to deploy your parachute before hitting the ground,
you will need to do it within those 4.5 seconds. Let's say 2 seconds,
plus or minus 2.
Or did you want it more accurate than that? You just said
"approximately how many seconds", so I thought this might be
accurate enough.