Bcrypt in PHP Timings

bcrypt-php-times.txt

To get an idea of the ideal number of bcrypt "rounds" to use for highest security without requiring
excessive CPU time, I wrote a little PHP script to hash a 9-character password with every possible
number of rounds (from 4 to 31), timing how long it takes to perform each hash.

This test was performed on an Intel Core i5-2500 CPU @ 3.3 GHz, running Windows 7 64-bit and 
PHP 5.4.0 (x86; VC9; CLI). The machine was being used for general development work at the same
time, but the script was only single-threaded and the development work wasn't particularly heavy,
so the times should be fairly accurate.

RESULTS (times in seconds):
 4:     0.0013420581817627
 5:     0.0025110244750977
 6:     0.0048818588256836
 7:     0.0094709396362305
 8:     0.019078016281128
 9:     0.037874937057495
10:     0.075153112411499
11:     0.15010213851929
12:     0.29720687866211
13:     0.59412312507629
14:     1.1954891681671
15:     2.4306519031525
16:     4.8433339595795
17:     9.7801411151886
18:    19.227061986923
19:    38.323782920837
20:    76.974308013916  ( 1.3 mins)
21:   156.51620221138   ( 2.6 mins)
22:   311.50990414619   ( 5.2 mins)
23:   625.07019400597   (10.4 mins)
24:  1247.1522209644    (20.8 mins)
25:  2493.5025999546    (41.6 mins)
26:  4999.0817778111    ( 1.4 HOURS)
27:  9997.7808241844    ( 2.8 HOURS)
28: 19863.30025506      ( 5.5 HOURS)
29: 39932.696026802     (11.1 HOURS)
30: [Probably 22 hours - I gave up at this point]
31: [Probably 44 hours]

And yes, the times appear to be exponential.