If you are talking about dissolved phase BTEX it  depends on such things as electron acceptor and nutrient availability but generally if only in the dissolved phase, BTEX will be degraded (and evaporated) quite quickly (from a well even quicker than from the main body of groundwater-hence the importance of purging the well before sampling). The problem is that if there is free product in contact with the groundwater or isolated in the unsaturated zone (but exposed to trickling rainwater) it will continuec to feed fresh BTEX into the groundwater untill it is depleted. This can last tens , or potentially hundreds, of years depending on how much free product is present. It is almost impossible to predict (in my opinion). 

Forensic analysis of petroleum products often looks for the dissapearance of BTEX as an indication of weathering (and thus trying to put a date on the spillage) but there is great uncertainty and you need to look at the isolated free product and not an extract of the aqueous phase.

I absolutely agree with Dr.  Alan P Newman. I have experience from treatment of contaminated aquifers with BTEX from gasoline. We treated plumes 50 years old and the composition of BTEX left dissolved in groundwater was different on different places according to redox potential, pH, concentration of electron akceptor and also according to microbial concsortia structure.

Regards Vit 

Here is a simple indicator parameter you can use.  Along a sampled plume from source to leading edge, you would expect to observe the ratio of B/T vs. B+T to decline as B+T declines...a sure sign of bacterially mediated intrinsic remediation (adsorption not considered).  Toluene and o-xylene decline most rapidly with distance from the source followed by m-, p-xylene, and benzene. Therefore, at a single site, you can just monitor B/T v B+T with periodic sampling to estimate remediation rates. B/T will not decline if the B+T ratio is affected by dilution from the multiple fluid sources that typically mix in a domestic well bore environment.  

Wiedemeier, T.H., Swanson, M.A., Wilson, J.T., Kampbell, D.H., Miller, R.N., and Hansen, J.E., 1996, Approximation of biodegradation rate constants for monoaromatic hydrocarbons (BTEX) in ground water: Ground Water Monitoring & Remediation, v. 16, p. 186–194, doi: 10.1111/j.1745-6592.1996.tb00149.x.

Thank you for your recommendation. I must have been asleep when I reviewed before sending this.  The B/T ratio would increase not decline, whereas the T/B ratio would decline with increasing attenuation. Sorry for the confusion. 

read this and see if it makes sense:

'Super bacteria' clean up after oil spills

Date:

March 10, 2014

Source:

SINTEF

Summary:

Researchers have achieved surprising results by exploiting nature's own ability to clean up after oil spills. Scientists know that marine bacteria can assist in cleaning up after oil spills. What is surprising is that given the right kind of encouragement, they can be even more effective.