It will heat up severely, that's why you need a huge heatsink and fan.
Use Ohm's law to calculate the power dissipation. Generally you can ignore surges unless they're really huge so just use the average current.
Once you have the maximum power, work out the maximum tolerable temperature rise.
What's the maximum ambient temperature?
What's the maximum junction temperature? See the datasheet.
ΔT = Ta - Tj
Now work out how good the heatsink needs to be.
Heatsinks are specified by thermal resistance which is measured in °C/W, a rating of 1°C/W means the heatsink's temperate will increase 1°C per Watt of power dissipated.
Add the thermal resistance of the junction to case to the number you calculated and any insulating tab you've used.
Now select a heatsink with a lower thermal resistance than the calculated value.