I let mine go a few months b/c I couldn't see any crack in the boots, but somehow the grease escaped anyways. So the boots looked good, but I had noise.
My GFs civic was the opposite, no noise, but both outer boots were completely ripped 360* around! If you can tackle it yourself, its not too hard of a job and you can get lifetime warrantied ones from kragen/autozone/etc for ~$60 an axle, and if it ever goes out - free replacement.