How to safely discharge a lead-acid battery?

Hook it up to a 60W headlamp bulb, that will take 5A. Car batteries usually have a capacity of around 45 to 60 AHr, if you assume the battery is fully charged and in good condition then it should take around 10 hours to discharge it.

I think that answers your question. But you should not fully discharge a lead-acid battery and leave it standing, you will permanently damage it.

What do you plan on doing with the battery? If you completely discharge it, it will modify the electrodes and you will have to recondition it to get it back up and running. If you really want to discharge it fully, get a large resistor. Did you know a light bulb makes a great resistor? You can use any material just as long as it will conduct reasonably and it won't heat up too much. Or use a light bulb. 40W lightbulb/ 12V = 3A

It appears that you have some erroneous, preconceived ideas about discharging a battery.
Specifically, if you want to fully discharge a typical car battery (12V, 60 A hr), all you need is a 20 ohm, 10 W resistor (or equivalent), and connect it across the battery terminals. Leave it connected for about 4 days, and with a voltmeter verify that the voltage is zero.
Since the current is only (12/20 =) 0.6A, no "heavy" wire is needed to connect the resistor.
Since the power being discharged is only (12 x .6 =) 7.2W, a 10W resistor should handle it with no problem.
Since it is drawing .6A, it should take (60/.6 =) 100 hours to discharge the battery.

The mentioned resistor should not cost more than $5 dollars. Good luck!