Allowing about half an hour from the inner city area (Cork) to Blarney, might even include looking for a place to park in Blarney.
There is a stone fenced Village Square/Green near the castle entrance, and though there is a narrow road (dare I say "lane") between the lowish stone wall of the Green and the buildings around it, it's park-able. especially right across from the police (Garda) station
Some people park in front of the supermarket, others leave their vehicle within the nearby grounds of the Blarney Woollen mills.
The last time we we visited Blarney & castle we did the woollen mills... nice stuff. The shop next to the supermarket had similar stuff but some items were nicer. We did the souvenirs shed near the castle gates. Again, somehow they've managed to be that bit different to the other two.
There is more to do within the castle than just kiss a stone, so, for example, if you wanted to explore the grounds, I'd suggest you allow at least 3 hours And now there's some house you can visit too, or so I've read somewhere.
Cork city itself... couple of hours at most. Excluding the amount of time I've recently heard it's taking to squeeze through the crowds to get yourself out of the English market, assuming you can get past the crowds to get in to it in the first place.