Depends how much you trust the trip to the airport. I used to be a 7 minute taxi/30 minute walk away, so I'd leave home at t-45, if the taxi didn't turn up I'd still have time, if it did turn up I had time for a coffee in the lounge.

Now I live further away it's much more variable. If I'm travelling from Heathrow T5 you have to be into security at t-35, so off the train at t-40, so out of Paddington at t-62, add a bit of leeway means leaving Paddington at t-80 to t-90 depending when the trains are due.

If it's a long haul where there only one flight a day I may well push it out to plan to arrive at an airport as early as t-75, which while excessive, is just in case there are problems like punctures or whatever.