Ah, well, I meant the time it takes for a single node to verify the vallidity of a transaction. Not the time it takes for the entire network to accept the transaction as valid and incorporate it into the blockchain. In other words, checking the signature, checking the structure of the transaction, checking the content of the transaction fields, all that, how long does that take for a node (assumptions can be made on processor power, etc)? This should be independent of the tx fee.
Depending on the speed of the host computer, it should not be more than a couple milliseconds.
I think in bitcoin core, the most time consuming part these days would be fetching tx's input(s) from UTXO database. Unless they'd happen to be in a cache.
Then verifying ECDSA signature(s) also needs some time. But this has been hugely improved by switching to sipa's secp265k1 lib.