I would aim for 2hr after a short nap, just slightly less than what you're usually going for. Spending 40min shush/patting a baby who's not going back to sleep is a bit futile at times, I'd err on the side of goving it a go for 5-10mins and if you think he'll go back to sleep, stick with it but if not, just get him up. If he was close to being tired enough when he went down, he should resettle pretty quickly. Sounds like he's quite UT for those naps if you can pat that long and he's never going back to sleep.