Someone with the knowledge (and desire) to compute the probability *could say* how often that occurs. It isn't that rare, ... maybe occurring once a week (wild-assed guess).

Maybe someone

*could* have already said... (or someone could search before starting a thread.):

So the chance a block won't be found after 10, 20 minutes, etc. (and the 1 in x chance)

>>> for x in range(10,131,10) :

print x, '%2.5f%%' % (100*math.exp(-x/10)), '%1.2f' % (math.exp(x/10))

10 36.78794% 2.72 (average block length)

20 13.53353% 7.39

30 4.97871% 20.09

40 1.83156% 54.60

50 0.67379% 148.41

60 0.24788% 403.43

70 0.09119% 1096.63

80 0.03355% 2980.96

90 0.01234% 8103.08

100 0.00454% 22026.47

110 0.00167% 59874.14

120 0.00061% 162754.79

130 0.00023% 442413.39And every day we can reasonably expect a block longer than 49 minutes, every week one longer than 69 minutes, every month 83 minutes:

>>> for x in [ 10*math.log(144), 10*math.log(144*7), 10*math.log(144*365.25/12)]:

print x, '%2.5f%%' % (100*math.exp(-x/10)), '%1.2f' % (math.exp(x/10))

49.6981329958 0.69444% 144.00

69.1572344863 0.09921% 1008.00

83.8548870042 0.02282% 4383.00So if hashrate maintains at a level as predicted by difficulty, you would expect a >60 minute block every three days (red), however if hashrate drops to 83% of the predicted rate, a >60 min block averaging one a day would not be unusual (orange) (don't forget, many long blocks one day and none for many days in a row also would not be unusual, variance tells us the unlikely is just as likely).

You can get a rough idea if the hashrate may be decreasing here:

http://dot-bit.org/tools/nextDifficulty.phpIt has both difficulty predictions for Bitcoin and Namecoin based on several periods. As Namecoin is also mainly merge-mined by Bitcoin miners, it can be a secondary indicator to evaluate the true direction of hashrate (as block finding, even over many blocks, is random, and can be non-predictive of actual hashrate).

In addition, I just asked myself "Does it matter if it's the weekend or not?". Well, here's the number of blocks found by day of the week for Jan 16 2012 (Monday) to Jan 13 2013 (Sunday); it's hard to assign any significance to any day: