It takes from 5 (shortest) to almost 40 (longest) minutes for `bitcoin-cli gettxoutsetinfo` to return an _empty_ dict. I noticed this first whilst configuring the Statoshi branch of Bitcoin Core, but went back to install Bitcoin Core (git clone -b v0.11.2 https://github.com/bitcoin/bitcoin.git) to find the same results.

```
20:10 pi@raspinode:~/bin/bitcoin.orig $ bitcoin-cli gettxoutsetinfo
{
}
20:12 pi@raspinode:~/bin/bitcoin.orig $ bitcoin-cli getinfo
{
    "version" : 110200,
    "protocolversion" : 70002,
    "blocks" : 393193,
    "timeoffset" : 0,
    "connections" : 12,
    "proxy" : "",
    "difficulty" : 113354299801.47113037,
    "testnet" : false,
    "relayfee" : 0.00005000,
    "errors" : ""
}
20:17 pi@raspinode:~/bin/bitcoin.orig $
```

Raspberry Pi 2 - 1GB memory - 4 core CPU 900MHz - Blockchain on SanDisk Ultra Android microSDXC 128GB up to 80 MB/Sec Class 10

All other RPC calls through bitcoin-cli are answered promptly and correctly.
See also issue on jlopp/statoshi https://github.com/jlopp/statoshi/issues/19#issuecomment-171350739

}
            stats.nSerializedSize += 32 + pcursor->GetValueSize();
            ss << VARINT(0);
        } else {
            return error("%s: unable to read value", __func__);
        }
        pcursor->Next();
    }
    stats.hashSerialized = ss.GetHash();
    stats.nTotalAmount = nTotalAmount;
    return true;
}
UniValue gettxoutsetinfo(const UniValue& params, bool fHelp)
{
    if (fHelp || params.size() != 0)
        throw runtime_error(
            "gettxoutsetinfo\n"
            "\nReturns statistics about the unspent transaction output set.\n"
            "Note this call may take some time.\n"
            "\nResult:\n"
            "{\n"
            "  \"height\":n,     (numeric) The current block height (index)\n"
            "  \"bestblock\": \"hex\",   (string) the best block hash hex\n"
            "  \"transactions\": n,      (numeric) The number of transactions\n"
            "  \"txouts\": n,            (numeric) The number of output transactions\n"
            "  \"bytes_serialized\": n,  (numeric) The serialized size\n"
            "  \"hash_serialized\": \"hash\",   (string) The serialized hash\n"
            "  \"total_amount\": x.xxx          (numeric) The total amount\n"
            "}\n"
            "\nExamples:\n"