4 years, as long as it's syphoned during minimally monthly WC, i see no problems, BUT if i were to set up another tank, i would go BB or just a 1/2" or so of sand, why complicate things w/ a DSB...? i like mine though, just requires more attention. make sure u have enough critters to stir it up a bit and keep it clean. some say syphoning a DSB is defeating the purpose, i say it's not, either have a filthy ticking time bomb DSB, or a clean DSB...
- this issue has been BEAT to death, so u will hear all different opinions...