I think it depends greatly on the brand (controller) and type of card. It takes about 40 seconds to clear a 10-shot buffer when using my 8GB Kingston class 4 SDHC card, and about 8 seconds to clear a 10-shot buffer when using my 16GB SanDisk Extreme (HD Video) Class 10 SDHC-I card (rated at 30MB/s, and comes darn close to that in testing on a few hardware review sites). It's important to note that the SDHC-I card uses the new UHS-I bus, which quadruples the potential bandwidth across the interface. (UHS-I cards are backwards compatible with older devices, but won't read or write at the rated speeds when used with devices that aren't UHS-I compliant. UHS-I cards are compatible with the D7000, but not listed as compatible for any Nikon cameras prior to the D7000.)
I suspect that card speeds will vary greatly across test devices, depending on the read/write performance of the device/camera as much as on the card itself, and of course also dependent on the bus interface. (I suspect that a high-speed UHS-I card probably has a better chance of meeting its rated speed than a card that isn't UHS-I. Actually I'm just about certain of that, since the SDHC bandwidth tops out at about 25MB/s with the "old" interface, but is capable of 104MB/s with the UHS-I interface.)
I'm happy with the performance of the Sandisk Extreme SDHC-I cards in the D7000. They cost me $45.99 per card through Amazon.
Hope this helps,
Keith