How many Q/A-Pairings in Question & Answer CAPTCHA are useful?

as i am using the Q&A-Captcha i wanted to know how much of these pairings are useful to prevent spam?
I have now about 7 sentences and ask for the first/second/third/etc. word in the appropriate sentence - which adds up to about 35 pairings in total.

Is this enough? Too much?
Any ideas will be appreciated...


Well-known member
Been using question & answer for over a year on local board not a single spammer is able to get in so far . I think updating them every few months makes lot of difference.