About every 40-50K. The problem was with the older style bolts and the new ones seem to have resolved most of the issues. There have been those on NC.org that have gone past 100k on the original bolts.
It is much easier to check and replace them before they break. Probably depends on how often you hit lift, I don't know if my wife ever revs her car up that high. My spyder gets into lift almost every day. As cheap as they are I would just replace them when going through the hassle of pulling them out.
I agree with dev, earlier 2zz engines are known for lift bolt failure. However I have 205k miles on my lift bolts in the celica. But I dont rev that car past 4k rpm much because it's my daily and I want it to last another 205k miles.