The heat should be there as soon as your engine gets warm, however long that takes. In an average car (is there such a think), I'd guessedamate that to be in the 3-5 minute range once the vehicle has started moving, or about 10 minutes if it's just idling (this also depends on how cold it is outside ... cold-cold, it'll take longer ... just chilly, not near as long). You should be able to look at the engine temperature gauge on the dash (if the Liberty is so equipped). When it gets up to temperature (usually the middle of the gauge for most vehicles), the air out of your vents should be hot at that point too.
If it never heats up, there are two things which come to mind. First, the heater core may be plugged. You can test the two hoses going into the passenger compartment from the engine itself. If both of these are hot when the fan blower is not blowing, the heater core is most likely in good shape. Secondly, if the thermostat is stuck in the open position. Usually, though, once the engine heats up, if the thermostat is stuck open, the air will be lukewarm. If the thermostat is stuck open, it would take the engine a long time to heat up (not just the heater core).
EDIT: I would like to add, what Solar Mike put in comments is correct as well. You need to double check to ensure there's no air in the system. If there's an air pocket in the wrong place, this could cause the heater core not to get warm, either.
