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How long does it take for the number to be checked in Prime95 if I have a number and would like to check it whether i'm correct. The largest is 58millions so I would like to put that exponent in Prime95 what is the expected time to run it on average i5 3.40GHz Windows 7 x64, and whether only one number can be check at once on one computer?

at this speed, one core working on one exponent (around 58 million) will take 20 to 22 day of work.
If you use 2 core, the time almost half : 13 to 16 days. If you add more more, the overall speed increase but not as much.
It is recommended, however to run only on real core.
below are example of runtime for a i5 2600, one core running

exponent;status; date of delivery ; time it took in days; residue; credit granted
[code]
50831701 C 2013-07-02 13:54 18.5 FCF5DE6D806489__ 93.1209
52023353 C 2013-06-19 19:28 18.8 81B92623E1A14C__ 99.6663
58692583 C 2012-12-16 07:53 20.6 C90E557C07CA47__ 124.3957
58957663 C 2012-12-16 12:49 20.8 11A45916ACE34E__ 130.9761
58957559 C 2012-12-16 14:14 20.9 A88D24DE34D16E__ 130.9759
59189587 C 2012-12-16 14:46 20.9 E3424E1FED28D8__ 131.4913
53626039 C 2013-04-04 15:43 21.5 BBA38A09F3694A__ 102.7367
51474433 C 2013-01-01 23:31 21.9 D8BC6CCBB9F1DE__ 94.2983
51497723 C 2013-01-01 23:59 21.9 45A8D568F02858__ 94.3410
57522991 C 2013-07-07 04:17 21.9 4682053E0FEC57__ 121.9168
51502453 C 2013-01-01 23:09 21.9 576D9D46CFBC7A__ 94.3496
57235931 C 2013-03-16 16:10 22.2 5446092C561E11__ 121.3084
51881251 C 2013-06-05 12:10 22.4 A27F388763E8BB__ 99.3940
58753129 C 2013-06-19 01:40 22.5 70D014BCF740F9__ 124.5240
52874599 C 2013-01-20 06:59 22.6 5CA6E6DC24B8A9__ 101.2971
58370539 C 2013-03-27 06:04 22.8 45003CAE7727E9__ 123.7131
54862487 C 2013-02-08 01:59 22.8 E919CEDB8D9A70__ 108.3915
54882799 C 2013-02-08 02:35 22.8 15ADF837E1A9D7__ 108.4316
59485639 C 2013-02-27 00:41 23.0 BF8B5539B772C0__ 132.1490
59634511 C 2013-07-02 13:13 23.0 EDB7F8F2014D39__ 132.4797
60663727 C 2013-02-27 08:57 23.4 0761FA123C3599__ 134.7662
53666939 C 2013-01-03 05:01 23.9 D8A5223B558454__ 102.8150
55696189 C 2013-02-08 09:43 23.9 79C02E28FCA470__ 118.0450
60994027 C 2013-03-18 16:50 24.0 DF747652C15C12__ 135.4999
56485783 C 2013-01-21 09:17 25.0 B95751148F7E5F__ 119.7185
56258959 C 2013-01-21 08:52 25.0 2DDFD2D900A622__ 119.2377
56228269 C 2013-01-21 07:51 25.0 0A196C7EC8EF9F__ 119.1727
53882251 C 2013-05-22 05:14 25.1 8EF79BB82468A1__ 106.4549
51068729 C 2013-04-16 08:45 25.2 DD51D02021114A__ 93.5551
61493321 C 2013-03-09 06:09 26.5 EB97C8D59F337C__ 144.4559
60644209 C 2013-04-29 09:07 26.6 06738331905537__ 134.7228
59104229 C 2013-05-02 10:00 27.8 E4155837D4D40C__ 131.3017
59808263 C 2013-06-01 01:01 28.4 3BBADC39CB3915__ 132.8657
61252231 C 2013-06-14 08:03 28.6 81F594798847A0__ 143.8896
58464271 C 2013-02-15 13:42 30.0 0884373D0CE11B__ 123.9118
61516321 C 2013-05-24 23:52 31.0 BC93337A816EB1__ 144.5100
62553431 C 2013-05-08 20:49 32.0 984467F8511053__ 146.9463
61933561 C 2013-04-13 10:18 32.7 D4FC878EB1B449__ 145.4901
[/code]
There is some iregularility; but it will give you an idea of the speed.

I'd bet Firejuggler gives you a better answer at the same time I'm typing this, but why don't you just fire the number up in prime95 and then wait a reasonable amount of time (say 30 minutes) then under the "test" menu there is a Status option which will give an approximate completion date. If you have a decent 4 core machine and set all cores running on the one number it will take a couple weeks. (I'm speculating.)

Thank you guys, so the virtual computer that GIMPS is running is not minimalizing the time, am I correct? and is there an official list of numbers that people already checked at least once?

The aim is to maximise throughput rather than the time taken for each workunit.

The server has a database of results, assignments etc. If you visit [url]www.mersenne.org[/url] and expand the section of the menu on the left labelled Results Queries, you can run various reports where you can put in an exponent or a range.