Well, at the airports I am most familiar with (LBA and MAN T1 and 3) at peak times and needing to check a bag, anything less than 90 minutes would feel risky to me. Half an hour at the check in desk, half an hour queueing for security and fifteen minutes to the gate has happened to me. It's not the average time that matters, it's the time you need to leave to feel comfortable about making it.
The cross over point at which air can get a significant market share seems to me to have gone out from three to four or even five hours rail journey time. MAN to Paris by train or plane--pretty marginal really.