p(10mn) = probability to find a block in slot 0-10mn = 1/2
p(20mn) = p(slot 10mn-20mn) = 1/4 ( = 1/2 coz not found previously * 1/2 coz found in this 10mn slot)
p(30mn) = 1/8 (= 1/2 * 1/2 * 1/2)
p(40mn) = 1/16
p(50mn) = 1/32
p(1 hour) = 1/64

so probability to "not find a block in one hour" = 1/64 ~ 0.01562 .. could happen a couple of times a day