A very long time - many years, decades ...
http://www.epa.gov/hpv/pubs/summarie...c/c16856rs.pdf
page 15 of pdf:
3.1.2 STABILITY IN WATER
Test Substance
Identity: Alkyl dimethyl benzyl ammonium chloride (CAS RN 68424-85-1; Alkyl (C12-16) dimethylbenzylammonium chloride)
...
Degradation %: No degradation determined (measured concentration of parent chemical was 96.3% ± 4.47% over a pH range of 5 to 9 at 25 °C after 30 days)
Half-life: An accurate estimate of the half-life for the hydrolysis could not be determined since no significant degradation of the test compound was detected during the 30-day evaluation period.
Conclusions: ADBAC was found to be hydrolytically stable in the pH range 5 to 9 at 25 °C.