user638: Assuming:

An average modern computer
Using a single thread, as the current Satoshi client does
Using the OpenSSL library, as the Satoshi Bitcoin client does
The signature is not in the signature cache

I speculate that the time may be between 1 and 10 msec.

user208: It depends a lot on the hardware, and on the architecture. OpenSSL use some assembly-optimized routines, which result in a speed factor of +- x2 between 32 bit and 64 bit (the latter is faster). That said, I've seen verifications taking around 0.47 ms on an Intel Xeon E5-1650 @ 3.20GHz, using 64-bit code. 
Also, version 0.8 will actually do signature verification in parallel. There is also a patch with an optimized ECDSA verifier for the specific curve we're using, which should give a 20% speedup (discovered by Hal Finney). This patch will probably not make it into v0.8, though.

user120: I instrumented the Satoshi client to see hwo long a transaction verification takes. This includes the signature verification, but is probably more of an upper bound as it also includes several disk seeks.

4500 Transactions. With the following percentiles:

50 percentile 5.0
75 percentile 10.0
85 percentile 15.0
95 percentile 36.0
99 percentile 207.27

While not at all representative it would point towards your estimate being correct.
Note: this was taken on a Lenovo T420 with spinning disks.

638: <p>Assuming:</p>

<ul>
<li>An average modern computer</li>
<li>Using a single thread, as the current Satoshi client does</li>
<li>Using the OpenSSL library, as the Satoshi Bitcoin client does</li>
<li>The signature is not in the signature cache</li>
</ul>

<p>I speculate that the time may be between 1 and 10 msec.</p>
 208: <p>It depends a lot on the hardware, and on the architecture. OpenSSL use some assembly-optimized routines, which result in a speed factor of +- x2 between 32 bit and 64 bit (the latter is faster). That said, I've seen verifications taking around 0.47 ms on an Intel Xeon E5-1650 @ 3.20GHz, using 64-bit code. </p>

<p>Also, version 0.8 will actually do signature verification in parallel. There is also a patch with an optimized ECDSA verifier for the specific curve we're using, which should give a 20% speedup (discovered by Hal Finney). This patch will probably not make it into v0.8, though.</p>
 120: <p>I instrumented the Satoshi client to see hwo long a transaction verification takes. This includes the signature verification, but is probably more of an upper bound as it also includes several disk seeks.</p>

<p><img src="https://i.stack.imgur.com/7IEMX.png" alt="Transaction verification time"></p>

<p>4500 Transactions. With the following percentiles:</p>

<ul>
<li>50 percentile 5.0</li>
<li>75 percentile 10.0</li>
<li>85 percentile 15.0</li>
<li>95 percentile 36.0</li>
<li>99 percentile 207.27</li>
</ul>

<p>While not at all representative it would point towards your estimate being correct.</p>

<p>Note: this was taken on a Lenovo T420 with spinning disks.</p>
