Only 5.26 minutes!

Availability %
Downtime per year
Downtime per month*
Downtime per week

90%
36.5 days
72 hours
16.8 hours

95%
18.25 days
36 hours
8.4 hours

98%
7.30 days
14.4 hours
3.36 hours

99%
3.65 days
7.20 hours
1.68 hours

99.5%
1.83 days
3.60 hours
50.4 min

99.8%
17.52 hours
86.23 min
20.16 min

99.9%
8.76 hours
43.2 min
10.1 min

99.95%
4.38 hours
21.56 min
5.04 min

99.99%
52.6 min
4.32 min
1.01 min

99.999%
5.26 min
25.9 s
6.05 s

99.9999%
31.5 s
2.59 s
0.605 s

References
http://en.wikipedia.org/wiki/5_nines