First Order Degradation Constant (λ)(d^-1)
Based on a bit more than a cursory search, there doesn't seem to be a first order degradation constant readily available for 1,3,5-trimethylbenzene (mesitylene) in the open literature. This leaves us with your second option, the Title 35 Illinois Administrative Code 740$^{[1]}$, Table A (herein referred to as Table A).  

By far the most similar compound to 1,3,5-trimethylbenzene that can be found in Table A, based on both chemical structure and physical properties, is the total xylenes (CAS # 1330-20-7).  This is actually a mixture of the three possible isomers of xylene.  

Both the images below and all physical and chemical properties given for the total xylenes were taken from this Wikipedia page unless otherwise stated. The 1,3,5-trimethylbenzene image and all of its chemical and physical properties were taken from here.


1,3,5-trimethylbenzene (mesitylene)  

 
In order: o-xylene, m-xylene, p-xylene   

Selected properties:
Mesitylene: density = 0.86 ($\mathrm{20^oC}$), B.P. = $\mathrm{165^oC}$, M.P. = $\mathrm{-45^oC}$, and water solbility = 50 mg/L at $\mathrm{25^oC}^{[2]}$
Xylenes : density = 0.86 ($\mathrm{20^oC}$), B.P. = $\mathrm{138^oC}$, M.P. = $\mathrm{-47^oC}$, and water solbility = 110 mg/L at $\mathrm{25^oC}^{[2]}$  

1) TITLE 35: ENVIRONMENTAL PROTECTION, SUBTITLE G: WASTE DISPOSAL, CHAPTER I: POLLUTION CONTROL BOARD PART 740, SITE REMEDIATION PROGRAM  

2) Yalkowsky, S.H., He, Yan., Handbook of Aqueous Solubility Data: An Extensive Compilation of Aqueous Solubility Data for Organic Compounds Extracted from the AQUASOL dATAbASE. CRC Press LLC, Boca Raton, FL. 2003