Let steep for as long as desired, then remove and discard. A half hour will result in a pleasant tingle. For truly hot firewhiskey, 4-5 hours is recommended. Of course, using more peppers will result in hotter firewhiskey in less time.
It's a good idea (if you're serving this to guests) to have two different strengths of firewhiskey so that those with more sensitive palates can still enjoy it.
To shorten the process, place peppers in a small amount of water and microwave for 45 seconds, and use this to flavor the drink. It may be less spicy but it's clearly quicker. You could also shred the ginger instead of slicing, but it's hard to remove it afterward.