It's been established over the years that two hours under normal conditions is considered promptly available. There is no hard limit because things happening such as accidents, traffic jams, flats or mechanical issues can impact drive times. If it takes you 4 hours on what is a normal two hour drive you will not face any discipline. If on the other hand you live 150 miles from the airport in a high traffic area and cause a late departure you might have a issue. This came to a head in the early nineties with commuters from San Diego sitting short call at home for LAX. One pilot was even terminated however got his job back on the basis he was not on short call but rather in the window we had then.