It is impossible to answer this accurately without knowing details such as the hardware the miner is using, their utxo storage method, and their validation process.
However, in general, the utxo set can be stored as a list keyed by the utxo identifier (
txid:vout). This provides O(1), or constant, look up complexity for a single utxo.
However, the utxo set may not fit entirely in memory on many systems, and the actual retrieval time may be longer than you'd expect in an ideal scenario after accounting for disk access time, and other implementation specific details.
In a miner's case, they validate incoming transactions and prepare a block template by feerate asynchronously from the actual mining operation - this allows them to have a reasonably good chance of maximising their earnings per block without having the mining process blocked by tx and block validation systems.