Before with my almost dead thermostat, it would take 20+ mins.
But after swapping over to a new one, it'll take roughly around 2-3kms @ 60km/h.
As mad082 said, it really depends on the weather as well. If it's a really hot day, then the thermostat will open a lot sooner versus a cold night where it'll stay closed for longer so the engine can warm up properly.