How much time it takes to verify a Bitcoin signature in the Satoshi client? 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.
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.